Summary

Fixes #5051

Temp state keys (prefix temp:) can contain non-serializable objects such as FunctionTool instances stored by _call_llm_node. _trim_temp_delta_state() already filters these for persistence in append_event(), but the SSE event generator serializes events before they reach that path, causing PydanticSerializationError when streaming=true.

This PR applies the same State.TEMP_PREFIX filtering in the SSE generator before model_dump_json().

Root cause

1. _call_llm_node.py:131 stores FunctionTool objects in ctx.state['temp:tools_dict']
2. State.delta is a reference to event_actions.state_delta — writes propagate immediately
3. base_session_service.py:140 (_trim_temp_delta_state) filters temp: keys, but only inside append_event()
4. adk_web_server.py:1918 serializes events via model_dump_json() before append_event() — temp: keys with non-serializable values are still present → 💥

Non-streaming (/run) works because events pass through the full append_event() pipeline before serialization.

Changes

• src/google/adk/cli/adk_web_server.py — Filter State.TEMP_PREFIX keys from state_delta in the SSE generator before model_dump_json() (mirrors _trim_temp_delta_state logic)
• tests/unittests/cli/test_fast_api.py — Regression test: event with non-serializable temp: state keys streams successfully, non-temp keys are preserved

Test plan

• New test test_agent_run_sse_filters_temp_state_keys verifies:
  • SSE response succeeds (no serialization crash)
  • Non-temp state keys (user_request) are preserved in the SSE output
  • Temp-scoped keys (temp:tools_dict, temp:other) are stripped
• All existing SSE tests pass (8/8):

tests/unittests/cli/test_fast_api.py::test_agent_run_passes_state_delta PASSED
tests/unittests/cli/test_fast_api.py::test_agent_run_passes_invocation_id PASSED
tests/unittests/cli/test_fast_api.py::test_agent_run_sse_splits_artifact_delta PASSED
tests/unittests/cli/test_fast_api.py::test_agent_run_sse_does_not_split_artifact_delta_for_function_resume PASSED
tests/unittests/cli/test_fast_api.py::test_agent_run_sse_yields_error_object_on_exception PASSED
tests/unittests/cli/test_fast_api.py::test_agent_run_sse_filters_temp_state_keys PASSED
tests/unittests/cli/test_fast_api.py::test_auto_creates_session[/run_sse] PASSED
tests/unittests/cli/test_fast_api.py::test_returns_404_without_auto_create[/run_sse] PASSED

🤖 This PR was developed with AI assistance (Claude Code).