fix(pi): preserve Antigravity state across turns

[asp345]

Problem

The Pi extension loses Antigravity state across multi-turn and tool-call requests:

• backend functionCall.id values are replaced and then omitted from replayed calls and responses
• streamed thinking and signatures attached to text-only responses are discarded
• signatures from a previous model are replayed after switching models
• historical function responses keep the same role after a model switch, unlike the native client
• request step metadata is derived from part count and omits last_execution_id
• GPT-OSS usage can arrive in an SSE frame after finishReason: STOP, but Pi cancels immediately

The missing tool IDs make Claude Opus reject the continuation after tool execution:

HTTP 400 INVALID_ARGUMENT
messages.1.content.0.tool_use.id: Field required

Fix

• preserve backend function-call IDs and replay matching IDs in functionCall and functionResponse
• carry a batch thought signature onto its first function call
• stream and retain Pi thinking content and signed visible text for same-model history
• strip thinking and signatures when history crosses model boundaries
• emit correlated foreign-model function responses with the role observed from native model-switch traffic
• track completed execution IDs in the shared request session and use content-based step counting as a Pi-only mode
• consume GPT-OSS usage sent immediately after STOP, with a bounded one-second wait
• explicitly return the SSE iterator before cancelling the response body to release the reader and socket

Trace verification

Additional captures were taken from one continued agy 1.1.5 conversation covering:

1. two text-only Claude Opus turns
2. a Claude Opus tool call and function-result continuation
3. a switch to Gemini 3.6 Flash
4. a switch to GPT-OSS 120B

The regression fixtures reproduce the captured execution-aware step sequence, same-model thinking/text-signature replay, tool-call signature placement, cross-model sanitation and roles, and GPT STOP-then-usage ordering.

Verification

• npm run build
• npm run typecheck
• npm test — 1,270 passed, 25 todo
• live Claude Opus tool loop completed successfully
• live same-model Opus follow-up accepted signed history
• live Opus → Gemini → GPT session completed successfully
• live GPT-OSS response reported nonzero terminal usage

PR #6 changes the same Pi adapter files but does not currently include these behaviors. They should be retained if that rewrite lands first.

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}