fix: Replace threading.local() with contextvars for async-safe A2A request context

[bokelley]

Problem

A2A server was using threading.local() to store request context (auth token and headers), which doesn't work properly with async/await code. Thread-local storage is thread-specific, but async functions can execute on different threads, causing context variables to become inaccessible in async handlers.

This manifested as 'No tenant context set' errors in list_authorized_properties and other A2A endpoints because:

1. Middleware set _request_context.request_headers in one thread
2. Async handler tried to read it via getattr(_request_context, 'request_headers', {})
3. Async handler was running in different thread → got empty dict
4. Tenant detection failed → get_current_tenant() raised RuntimeError

Solution

Replace threading.local() with contextvars.ContextVar, which properly handles async contexts:

Before:
```python
_request_context = threading.local()
_request_context.auth_token = token
_request_context.request_headers = dict(request.headers)
token = getattr(_request_context, 'auth_token', None)
```

After:
```python
_request_auth_token: contextvars.ContextVar[str | None] = contextvars.ContextVar('request_auth_token', default=None)
_request_headers: contextvars.ContextVar[dict | None] = contextvars.ContextVar('request_headers', default=None)
_request_auth_token.set(token)
_request_headers.set(dict(request.headers))
token = _request_auth_token.get()
```

Changes

Production Code

• src/a2a_server/adcp_a2a_server.py: Replaced threading.local() with two ContextVars
  • Updated all 5 access points to use .get() and .set() methods
  • Updated middleware cleanup to use .set(None) instead of delattr()
  • Added explanatory comment about ContextVars working with async code

Test Code

• tests/integration/test_a2a_response_message_fields.py: Updated 1 test fixture
• tests/integration_v2/test_a2a_skill_invocation.py: Updated 20 tests
• All tests now use adcp_a2a_server._request_headers.set() instead of mocking

Schemas

• Updated cached AdCP schemas (create/update media buy requests) to latest versions

Testing

• ✅ All 20 A2A integration tests pass
• ✅ No breaking changes - ContextVars are drop-in replacement for threading.local()
• ✅ All pre-commit hooks pass
• ✅ Fixes 'No tenant context set' error in list_authorized_properties A2A calls

References

• Python ContextVars docs: https://docs.python.org/3/library/contextvars.html
• Issue: 'Error loading properties: A2A agent returned error: Unable to retrieve authorized properties'

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com