fix: restore event metadata in A2A inbound converters

[giulio-leone]

Summary

Fixes #5185 — A2A inbound converters now restore custom_metadata, usage_metadata, and error_code from A2A message metadata, and metadata-only messages no longer return None.

Root Cause

The outbound converter (from_adk_event.py) serializes event metadata fields (custom_metadata, usage_metadata, error_code, etc.) into A2A metadata using the adk_ key prefix. The inbound converter (to_adk_event.py) only extracted actions — all other fields were silently dropped.

Additionally, _create_event() returned None when there were no parts and no actions, even if event metadata was present, causing metadata-only messages to be discarded.

Changes

src/google/adk/a2a/converters/to_adk_event.py

• _extract_event_metadata() (new): Extracts custom_metadata, error_code, and usage_metadata from A2A metadata, mirroring what _add_event_metadata() serializes outbound.
• _create_event(): Accepts new optional params (custom_metadata, error_code, usage_metadata), sets them on the Event, and fixes the None-gate to preserve events that carry only metadata.
• All four converter functions (convert_a2a_message_to_event, convert_a2a_task_to_event, convert_a2a_status_update_to_event, convert_a2a_artifact_update_to_event) now extract and forward event metadata.

tests/unittests/a2a/converters/test_to_adk.py

• 9 new regression tests covering:
  • Metadata-only message returns valid Event (not None)
  • custom_metadata restoration (standalone and with content)
  • error_code restoration
  • usage_metadata restoration
  • All metadata fields together with actions
  • Empty message still returns None (existing behavior preserved)
  • Task, status update, and artifact update metadata restoration

Test Results

All 151 converter tests pass (14 pre-existing + 9 new + 128 others).

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