fix: migrate Gemini Interactions emitter to SDK 2.x event protocol (#277) (#279)

## Summary

Migrates the Gemini **Interactions** mock from the SDK 1.x
event/response format to SDK 2.x (the "Interactions breaking changes,
May 2026" shapes in `@google/genai` v2) — covering **both** the streamed
SSE emitter **and** the non-streaming (unary) JSON response builders.
Previously the streamed path produced
`interaction.start`/`interaction.complete` +
`content.start`/`content.delta`/`content.stop` (**zero `event_type`
overlap** with the v2 adapter — a migrated consumer hit its `switch`
default for every event and rendered an empty assistant message), and
the non-streaming path emitted a 1.x `outputs` envelope the v2 consumer
never reads. Both paths now speak v2.

Closes #277.

## What changed — streamed SSE

**`src/gemini-interactions.ts`** — rewrote the three SSE builders:
- Lifecycle: `interaction.start`/`complete` →
`interaction.created`/`completed` (id stays populated on both; status
text→`completed`, tool calls→`requires_action`).
- Text: `content.start/delta/stop` → `step.start { step: { type:
"model_output" } }` / `step.delta { delta: { type: "text", text } }` /
`step.stop`.
- Tool calls: call identity (`id`, `name`) now lives on `step.start`
with an `arguments: {}` placeholder; arguments stream as `step.delta {
delta: { type: "arguments_delta", arguments: "<json-string fragment>" }
}`, valid JSON by `step.stop`.
- `writeGeminiInteractionsSSEStream` now counts `step.delta` (not
`content.delta`) for the `truncateAfterChunks` budget.

**`src/stream-collapse.ts`** — `collapseGeminiInteractionsSSE` now
parses 2.x events (assemble tool calls from `step.start` identity +
`arguments_delta` string fragments keyed by index; nested
`thought_summary.content.text`), while keeping 1.x parsing for
previously recorded fixtures. Hardened against silent data loss:
malformed assembled args and index-less / uncorrelated
`step.start`/`arguments_delta` are flagged via
`droppedChunks`/`firstDroppedSample` rather than written silently.

## What changed — non-streaming (unary) responses

The `@google/genai` v2 Interactions consumer
(`extractTextFromInteraction` in
`packages/ai-gemini/src/experimental/text-interactions/adapter.ts`,
TanStack/ai#781) reads a non-streaming interaction from
`interaction.output_text` (string fast-path) then walks
`interaction.steps` — `model_output` steps' `content[]` text parts and
`function_call` steps — and **never** reads `outputs`. So a v2 consumer
doing a non-streaming completion got a silently empty result: the same
failure class the streamed path fixed. These builders are reachable (`if
(!streaming)` routing in `src/gemini-interactions.ts`).

Migrated `outputs → steps`:
- `buildInteractionsTextResponse` → `{ id, status: "completed", model,
role: "model", output_text: content, steps: [{ type: "model_output",
content: [{ type: "text", text: content }] }], usage }`.
- `buildInteractionsToolCallResponse` → `steps: toolCalls.map(... {
type: "function_call", id, name, arguments: <parsed> })`, status
`requires_action`; preserves the malformed-args `try/catch` +
`logger.warn` guard (extracted into a shared `buildFunctionCallStep`
helper).
- `buildInteractionsContentWithToolCallsResponse` → `output_text` + a
`model_output` text step followed by `function_call` steps.

## Tests + drift baseline

Updated unit/integration/collapse tests and
`src/__tests__/drift/sdk-shapes.ts`
(`geminiInteractionsStreamEventShapes`,
`geminiInteractionsResponseShape`,
`geminiInteractionsToolCallResponseShape`) to 2.x; added round-trip,
multiple/interleaved tool calls, ordering, malformed-args, usage/status,
and 1.x backward-compat coverage; flipped the non-streaming
unit/integration assertions to the v2 `steps`/`output_text` shape.

## Verification

```bash
grep -E 'event_type: "(step|interaction\.created|interaction\.completed)' dist/gemini-interactions.js
```
matches after build (0 legacy shapes remain). Full suite: **3982 passing
/ 44 skipped**, prettier + eslint clean, build OK.

Red → green proof for the non-streaming migration (assertions run
against the real builder output / real server JSON, not a
re-implementation):

```text
# RED (against the old `outputs` source, before the builder change)
 × response builders > builds text response
   → expected undefined to be 'Hello!' // Object.is equality
 × Gemini Interactions — non-streaming > returns tool call response
   → Target cannot be null or undefined.
       Tests  2 failed | 97 skipped (99)
 # (full file pre-fix: 15 failed | 84 passed (99))

# GREEN (after migrating the builders to `steps`)
 ✓ src/__tests__/gemini-interactions.test.ts (99 tests | 97 skipped)
       Tests  2 passed | 97 skipped (99)
 # (full file: 99 passed (99); full suite: 3982 passed | 44 skipped; eslint + prettier clean; build OK)
```

This unblocks re-enabling the `stateful-interactions` e2e test in
TanStack/ai#781 once aimock is bumped/released.

## Notes / scope
- One ambiguous case is deliberately not signalled: a streamed
`function_call` whose `step.start` carries an empty `{}` placeholder but
never receives an `arguments_delta` finalizes to `"{}"`. On the wire
that is byte-identical to a legitimately empty-args call, so flagging it
would false-positive on every legitimate empty-args call.
- Empty-text non-streaming responses emit `output_text: ""` and a single
`model_output` step whose text part is `""` (mirrors the streamed
empty-content behavior).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: jpr5

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Gemini Interactions mock now emits the SDK 2.x event protocol on both paths — streamed SSE (`step.*`, `interaction.created`/`completed`, tool args via `arguments_delta`) and non-streaming responses (`steps`/`output_text`); legacy 1.x recorded fixtures still parse (#279)
+
 ## [1.33.0] - 2026-06-23
 
 ### Added

src/__tests__/drift/sdk-shapes.ts

@@ -1587,7 +1587,8 @@ export function geminiInteractionsResponseShape(): ShapeNode {
     status: "completed",
     model: "gemini-2.5-flash",
     role: "model",
-    outputs: [{ type: "text", text: "Hello!" }],
+    output_text: "Hello!",
+    steps: [{ type: "model_output", content: [{ type: "text", text: "Hello!" }] }],
     usage: { total_input_tokens: 0, total_output_tokens: 0, total_tokens: 0 },
   });
 }
@@ -1598,7 +1599,7 @@ export function geminiInteractionsToolCallResponseShape(): ShapeNode {
     status: "requires_action",
     model: "gemini-2.5-flash",
     role: "model",
-    outputs: [
+    steps: [
       {
         type: "function_call",
         id: "call_abc123",
@@ -1613,43 +1614,43 @@ export function geminiInteractionsToolCallResponseShape(): ShapeNode {
 export function geminiInteractionsStreamEventShapes(): SSEEventShape[] {
   return [
     {
-      type: "interaction.start",
+      type: "interaction.created",
       dataShape: extractShape({
-        event_type: "interaction.start",
+        event_type: "interaction.created",
         interaction: { id: "int_abc123", status: "in_progress" },
         event_id: "evt_1",
       }),
     },
     {
-      type: "content.start",
+      type: "step.start",
       dataShape: extractShape({
-        event_type: "content.start",
+        event_type: "step.start",
         index: 0,
-        content: { type: "text" },
+        step: { type: "model_output" },
         event_id: "evt_2",
       }),
     },
     {
-      type: "content.delta",
+      type: "step.delta",
       dataShape: extractShape({
-        event_type: "content.delta",
+        event_type: "step.delta",
         index: 0,
         delta: { type: "text", text: "Hello" },
         event_id: "evt_3",
       }),
     },
     {
-      type: "content.stop",
+      type: "step.stop",
       dataShape: extractShape({
-        event_type: "content.stop",
+        event_type: "step.stop",
         index: 0,
         event_id: "evt_4",
       }),
     },
     {
-      type: "interaction.complete",
+      type: "interaction.completed",
       dataShape: extractShape({
-        event_type: "interaction.complete",
+        event_type: "interaction.completed",
         interaction: {
           id: "int_abc123",
           status: "completed",
@@ -1664,48 +1665,51 @@ export function geminiInteractionsStreamEventShapes(): SSEEventShape[] {
 export function geminiInteractionsToolCallStreamEventShapes(): SSEEventShape[] {
   return [
     {
-      type: "interaction.start",
+      type: "interaction.created",
       dataShape: extractShape({
-        event_type: "interaction.start",
+        event_type: "interaction.created",
         interaction: { id: "int_abc123", status: "in_progress" },
         event_id: "evt_1",
       }),
     },
     {
-      type: "content.start",
+      type: "step.start",
       dataShape: extractShape({
-        event_type: "content.start",
+        event_type: "step.start",
         index: 0,
-        content: { type: "function_call" },
+        step: {
+          type: "function_call",
+          id: "call_abc123",
+          name: "get_weather",
+          arguments: {},
+        },
         event_id: "evt_2",
       }),
     },
     {
-      type: "content.delta",
+      type: "step.delta",
       dataShape: extractShape({
-        event_type: "content.delta",
+        event_type: "step.delta",
         index: 0,
         delta: {
-          type: "function_call",
-          id: "call_abc123",
-          name: "get_weather",
-          arguments: { city: "Paris" },
+          type: "arguments_delta",
+          arguments: '{"city":"Paris"}',
         },
         event_id: "evt_3",
       }),
     },
     {
-      type: "content.stop",
+      type: "step.stop",
       dataShape: extractShape({
-        event_type: "content.stop",
+        event_type: "step.stop",
         index: 0,
         event_id: "evt_4",
       }),
     },
     {
-      type: "interaction.complete",
+      type: "interaction.completed",
       dataShape: extractShape({
-        event_type: "interaction.complete",
+        event_type: "interaction.completed",
         interaction: {
           id: "int_abc123",
           status: "requires_action",