fix: preserve JSON-native types in A2A metadata serialization

[YizukiAme]

Fixes #5183

Problem

_serialize_value() in from_adk_event.py falls through to str() for JSON-native Python types (dict, list, bool, int, float), corrupting structured metadata values into their string representations during A2A event conversion.

For example, a custom_metadata dict like {"retry": True, "max_attempts": 3} gets serialized as "{'retry': True, 'max_attempts': 3}" — a Python repr string instead of a proper JSON object.

Fix

• Preserve JSON-native types directly: dict, list, str, bool, int, float are now returned as-is since they are already valid JSON values.
• JSON normalization fallback: For other JSON-serializable objects (e.g., custom classes with __dict__), normalize via json.loads(json.dumps(value)) before falling back to str().
• Added regression test for structured custom_metadata round-trip to ensure values are not corrupted to strings.

Logs After Fix

$ uv run pytest tests/unittests/a2a/converters/test_event_round_trip.py -v

tests/unittests/a2a/converters/test_event_round_trip.py::test_custom_metadata_structured_roundtrip PASSED

========== 143 passed, 124 warnings in 9.12s ==========

Testing Plan

uv run pytest tests/unittests/a2a/converters/ -v

All 143 A2A converter tests pass. The new test_custom_metadata_structured_roundtrip test verifies that dict, list, bool, int, and float values survive the _serialize_value() → metadata round-trip without being corrupted to strings.