Fix stale auth context in stateful StreamableHTTP sessions

[raashish1601]

Summary

• rebind the auth ContextVar from the current transport request before low-level server dispatch
• add a stateful StreamableHTTP regression that switches from token-A to token-B within one session
• keep the existing request-context propagation behavior covered while fixing get_access_token() for later requests
  Closes get_access_token() returns stale token in stateful streamable-HTTP sessions #2208.

Testing

• .\.venv\Scripts\python -m pytest tests\server\auth\middleware\test_auth_context.py tests\server\auth\middleware\test_auth_context_streamable_http.py -q
• .\.venv\Scripts\python -m pytest tests\shared\test_streamable_http.py -q -k "request_context_propagation or request_context_isolation"