fix(streaming): preserve SSE tool calls in session history

[kiwigitops]

Summary

• preserve tool-call parts in non-progressive SSE final aggregated responses
• keep generated function-call ids and add a regression covering text plus multiple tool calls

Closes #355.

Testing

• npm run build --workspace core
• npx vitest run --project unit:core core/test/utils/streaming_utils_test.ts

[kiwigitops]

Updated after the latest CI failure. Non-progressive SSE now flushes accumulated text and emits tool-call parts once, instead of returning the calls again from close(). That keeps the calls saveable in session history without re-invoking them from the final aggregate.

Verified:

• npm run build
• npx vitest run --project unit:core core/test/utils/streaming_utils_test.ts core/test/models/google_llm_test.ts
• npx vitest run --project integration tests/integration/a2a/basic/a2a_agent_test.ts

[kiwigitops]

Rebased onto current main after the Windows validation failure. The stale lint errors from the previous merge base are gone on this branch now.

Checks after rebase:

• npm run build
• npx vitest run --project unit:core core/test/utils/streaming_utils_test.ts core/test/models/google_llm_test.ts
• npm run lint
• git diff --check