fix(grpc): preserve null values in protobuf Struct (#578)

## Summary

- update the ts-proto generator from v2.6.1 to v2.11.0 so generated
`google.protobuf.Struct` wrappers preserve JavaScript `null`
- encode JavaScript `null` as `google.protobuf.NullValue.NULL_VALUE`
- decode the protobuf `nullValue` case back to JavaScript `null`
- add a wire-level regression test covering direct, nested, and array
metadata values

## Root cause

The generated `Value.wrap()` implementation assigned the literal
JavaScript `null` to a protobuf enum field. `@bufbuild/protobuf`
requires an integer enum value, so gRPC serialization failed with
`invalid int32: object`.

The reverse conversion also returned the enum value `0` instead of
restoring JSON `null`. The newer ts-proto output corrects both
directions so `google.protobuf.Struct` fields preserve JSON semantics
across the gRPC wire.

## Impact

Tasks, messages, and other A2A gRPC payloads can now contain valid JSON
`null` values in metadata or other `Struct`-backed fields without
crashing or changing them to `0` after deserialization.

## Target branch

This PR is rebased onto and directly targets `epic/1.0_breaking_changes`
for the stable 1.0.0 SDK work.

## Generation note

The checked-in 1.0 bindings contain post-generation adjustments. A full
`buf generate` currently rewrites those bindings and adds unrelated
generated files, so this PR refreshes only the affected `Struct`
artifact together with the generator pin to keep the fix scoped.

## Validation

- `npm test` — 66 files, 1,375 tests passed
- `npm run test:edge` — 49 files, 1,081 tests passed, 2 skipped
- `npm run build` — passed
- `npm run test-build` — passed
- `npm run lint:ci` — passed in GitHub Actions

Fixes #576

Author: MyPrototypeWhat

src/grpc/buf.gen.yaml

@@ -11,7 +11,7 @@ inputs:
       - google/rpc/status.proto
       - google/rpc/error_details.proto
 plugins:
-  - remote: buf.build/community/stephenh-ts-proto:v2.6.1
+  - remote: buf.build/community/stephenh-ts-proto:v2.11.0
     opt: 
       - oneof=unions-value
       - importSuffix=.js
@@ -21,4 +21,4 @@ plugins:
       - outputJsonMethods=false
       - outputPartialMethods=false
       - useDate=string
-    out: ./pb
+    out: ./pb

src/grpc/pb/google/protobuf/struct.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest';
+import { A2AServiceService, SendMessageResponse } from '../../../src/grpc/pb/a2a.js';
+
+describe('gRPC protobuf serialization', () => {
+  it('preserves null values in task metadata', () => {
+    const response: SendMessageResponse = {
+      payload: {
+        $case: 'task',
+        value: {
+          id: 'task-1',
+          contextId: 'context-1',
+          status: undefined,
+          artifacts: [],
+          history: [],
+          metadata: {
+            direct: null,
+            nested: { value: null },
+            items: [null, 'present'],
+          },
+        },
+      },
+    };
+
+    const serialized = A2AServiceService.sendMessage.responseSerialize(response);
+    const deserialized = A2AServiceService.sendMessage.responseDeserialize(serialized);
+
+    expect(deserialized).toEqual(response);
+  });
+});