fix: Retry ClosedResourceError on isolated session

[elainegan-openai]

Summary

• make ClosedResourceError trigger isolated-session retry for shared-session MCP tool calls
• add focused regression coverage for the retry classifier

Testing

• ruff check src/agents/mcp/server.py tests/mcp/test_client_session_retries.py
• PYTHONPATH=/Users/elainegan/code/openai-agents-python/src pytest -q tests/mcp/test_client_session_retries.py -k "closed_resource_on_isolated_session or retries_5xx_on_isolated_session or retries_cancelled_request_on_isolated_session or does_not_retry_4xx_on_isolated_session"