fix: preserve JSON-native types in A2A _serialize_value()

[enjoykumawat]

Summary

Fixes #5183

_serialize_value() in from_adk_event.py was calling str() on all non-Pydantic values, which corrupted JSON-native metadata types (dict, list, int, float, bool) by converting them to string representations.

Before: {"count": 42} became {"count": "42"}, {"tags": ["a","b"]} became {"tags": "['a', 'b']"}

After: JSON-native types (dict, list, int, float, bool, str) are passed through as-is. Only non-JSON-serializable types (e.g., datetime) are stringified.

Changes

• src/google/adk/a2a/converters/from_adk_event.py: Added isinstance check for JSON-native types before the str() fallback in _serialize_value()
• tests/unittests/a2a/converters/test_from_adk.py: Added TestSerializeValue class with tests for dict, list, int, float, bool, str, None, and non-JSON type handling

Test plan

• All existing tests pass (pytest tests/unittests/a2a/converters/test_from_adk.py — 12 passed)
• New tests verify each JSON-native type is preserved as-is
• New test verifies non-JSON types (e.g., datetime) are still stringified