MCP auth retry leaves orphaned transport/session when 401 occurs during initialize

Start in api/core/mcp/auth_client.py, tracing _execute_with_retry, enter, _initialize, and connect_server, then inspect the repro assertion for the partial-session case. Verify that a 401 during initialization closes the existing ExitStack and clears session state before retrying, while the fully initialized path still retries correctly.

Problem

MCPClientWithAuthRetry._execute_with_retry only tears down the MCP transport/ClientSession ExitStack when self._initialized is already True.

MCPClient.__enter__ sets _initialized = True after _initialize() returns. Inside _initialize → connect_server, the streamable-HTTP/SSE transport and ClientSession are already pushed onto _exit_stack before ClientSession.initialize() runs. A 401 from that handshake raises MCPAuthError while _initialized is still False.

On auth refresh + retry, the client therefore:

1. Updates Authorization headers
2. Calls _initialize() again without closing the first ExitStack entries
3. Opens a second transport + session while the expired-token connection stays alive until the outer context exits

Why this matters

• Core MCP OAuth / session-binding path used by tool list + invoke
• Leaks concurrent MCP sessions (servers that bind one session per client can reject or confuse the second connect)
• Stale connection with the old token remains open alongside the refreshed one

Evidence

# Current gate in api/core/mcp/auth_client.py
if self._initialized:
    self._exit_stack.close()
    self._session = None
    self._initialized = False
    self._initialize()
    self._initialized = True

Repro unit assertion (fails on main): after MCPAuthError with _initialized is False and a partial _session already attached, _exit_stack.close() is never called before the retry.

Proposed fix

Always close _exit_stack, clear _session, reset _initialized, and replace the ExitStack before retrying. Only call _initialize() again when the client was already fully initialized (list_tools / invoke path). The __enter__ retry path re-enters and initializes itself.

Related (not the same root cause)

• #41499 / #41482 — orphaned JSON-RPC responses crashing the receive loop (base_session.py)
• #42326 / #42327 — invoke skipping OAuth when custom headers are set (mcp_tool/tool.py)