OpenAI Agents Python SDK v0.19.4 fixes guardrails, sessions, and MCP

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OpenAI released v0.19.4 of its open-source Python agents framework, a bug-fix focused patch addressing 16 issues across guardrails, sessions, tracing, sandbox, and MCP (Model Context Protocol — the emerging standard for agent-to-tool communication). Key fixes include preserving completed tool guardrail results, correctly surfacing content-filter refusals in non-streaming mode, cancelling sibling tasks after concurrent failures, and enforcing token output budgets in the sandbox environment. Session management received several repairs: enforcing closed-state handling in MongoDB-backed sessions, preserving conversation history provenance, and rejecting reused branch IDs. The MCP transport layer also got improved retry backoff logic for streamable HTTP connections.