fix(mcp): reset stdio session stack state on cleanup

[OiPunk]

Summary

This PR hardens MCP server teardown by resetting _MCPServerWithClientSession state after cleanup().

Changes:

• Reinitialize exit_stack to a fresh AsyncExitStack after every cleanup.
• Clear server_initialize_result during cleanup, alongside session.
• Add a regression test that verifies cleanup resets stack state and that reconnecting with the same MCPServerStdio instance works reliably.

Why

When cleanup reuses the same exit stack object across connection lifecycles, callbacks/references from prior connection attempts can be retained longer than needed, especially after recoverable cleanup errors. Resetting teardown state makes MCP stdio lifecycle behavior deterministic and reduces risk of lingering subprocess/stream resources.

Testing

• uv run ruff check src/agents/mcp/server.py tests/mcp/test_connect_disconnect.py
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy -u NO_PROXY -u no_proxy uv run pytest tests/mcp/test_connect_disconnect.py tests/mcp/test_server_errors.py -q
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy -u NO_PROXY -u no_proxy uv run python -m trace --count --coverdir /tmp/openai_agents_618_trace --module pytest tests/mcp/test_connect_disconnect.py -q

Issue: #618

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

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Reviewed commit: 8cbf14ab51

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[OiPunk]

Good catch. You were right that grouped cancellation could still reset the stack.

I pushed d7f3e09 to handle this explicitly:

• Detect asyncio.CancelledError recursively inside BaseExceptionGroup.
• Mark cleanup as cancelled and re-raise when grouped cancellation is present.
• Keep exit_stack/session/server_initialize_result intact on that path so a follow-up cleanup can finish unwinding.
• Added regression test test_cleanup_grouped_cancellation_preserves_exit_stack_for_retry.

Local verification:

• uv run --with ruff ruff check src/agents/mcp/server.py tests/mcp/test_connect_disconnect.py
• uv run --with pytest pytest -q tests/mcp/test_connect_disconnect.py
• uv run --with mypy mypy src/agents/mcp/server.py tests/mcp/test_connect_disconnect.py
• uv run python -m trace --count --coverdir /tmp/openai_agents_2461_fixcr_trace --module pytest tests/mcp/test_connect_disconnect.py -q (new grouped-cancellation branch executed)

[OiPunk]

Follow-up: CI lint asked for formatting only. I pushed 928f113 (no logic changes) to satisfy ruff format check.