fix(events): serialize deferred Pydantic state models

[andrewrfitz]

Link to Issue or Description of Change

1. Link to an existing issue:

• Closes: GoogleSearchAgentTool parallel responses fail to serialize deferred GroundingMetadata (MockValSer) #6848
• Related: fix(sessions): prevent PydanticSerializationError when session state contains non-serializable objects #4748

Problem:

A Pydantic model configured with defer_build=True can retain a deferred
MockValSer after nested validation. When such a model is stored under
EventActions.state_delta, duck-typed serialization fails before the existing
unknown-value fallback can run. This aborts parallel function-response merging
when propagated grounding metadata is present.

Solution:

When normal state serialization fails, recursively build serializers for
Pydantic models in the state container and retry the original handler. Retrying
the handler preserves the caller's aliases and include/exclude directives. If
serialization still fails, the existing JSON-sanitizing fallback remains
unchanged for genuinely unsupported values such as callables.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally (see documented baseline failures below).

Passing checks:

• pytest tests/unittests/events/test_event_actions.py tests/unittests/flows/llm_flows/test_functions_simple.py -q
  • 95 passed
• pytest tests/unittests/events/test_event_actions.py -q under Python 3.10,
  3.11, 3.12, 3.13, and 3.14
  • 17 passed on each version
• mypy src/google/adk/events/event_actions.py
  • Success: no issues found in 1 source file
• pre-commit run --files src/google/adk/events/event_actions.py tests/unittests/events/test_event_actions.py
  • all hooks passed
• uv build
  • source distribution and wheel built successfully

Full-suite evidence:

• pytest tests/unittests -n auto -q: 12692 passed; the only two failures
  were both cases of
  test_entry_point_loads_only_allowlisted_packages, because the current
  locked environment eagerly loads httpx2.
• The same two failures reproduce on a detached, unmodified origin/main
  worktree.
• tox -p auto ran the full suite under Python 3.10 through 3.13. Each version
  had only those same two baseline failures, with 12,627–12,636 tests passing.
• Python 3.14 tox setup is currently blocked because locked scipy==1.15.3
  has no compatible wheel and requires a Fortran compiler. The affected test
  file passes in an isolated Python 3.14.6 environment using the project's
  default dependencies.

Manual End-to-End (E2E) Tests:

• Ran the no-network parallel merge reproduction from GoogleSearchAgentTool parallel responses fail to serialize deferred GroundingMetadata (MockValSer) #6848 with
  google-genai==2.19.0.

Before the merge, the nested grounding model had a MockValSer. After this
change, the merge completes, its serializer is a SchemaSerializer, and the
merged state preserves both values:

serializer_before=MockValSer
serializer_after=SchemaSerializer
{'temp:_adk_grounding_metadata': {'webSearchQueries': ['some query']},
 'temp:other': 1}

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally with my changes (two unrelated
  baseline failures are documented above).
• I have manually tested my changes end-to-end.
• No downstream dependency changes are required.

Additional context

The regression test defines a fresh deferred Pydantic model and obtains the
nested value through parent-model validation. This avoids an order-dependent
test that could pass accidentally after another test has already built
GroundingMetadata's class serializer.