Python: persist pending request IDs through session serialization (fixes #5818)

[hanhan761]

Summary

When a workflow pauses for human-in-the-loop input (e.g. Magentic plan review), the pending request IDs are kept in \WorkflowAgent._pending_requests. If the session is serialized with \ o_dict()\ and later restored with \rom_dict()\ on a new agent instance, the pending requests are lost. The agent then starts a fresh run instead of resuming the paused workflow.

Changes

Added two helper methods on \WorkflowAgent:

• _store_pending_requests_in_session()\ — saves pending request IDs to \session.state["_pending_requests"]\ after each run
• _restore_pending_requests_from_session()\ — restores pending request IDs from \session.state\ before each run

Both _run_impl\ and _run_stream_impl\ call these methods, so the fix covers streaming and non-streaming execution.

Since the data is stored in \session.state, the existing \ o_dict()/\rom_dict()\ serialization handles it automatically — no changes needed to \AgentSession.

Related

Fixes #5818