Python: release functional workflow stream on agent response close - #10220

Motivation & Context

Closing a streaming FunctionalWorkflowAgent response leaves its owned workflow marked as running. The next sequential agent.run() raises RuntimeError: Workflow is already running, even after await response.close() returns. This affects cancellation or disconnect cleanup followed by another turn.

Description & Review Guide

• What are the major changes? Register workflow_stream.close as a cleanup hook on the outer ResponseStream, using the existing cleanup lifecycle to close the owned stream and release the run guard.
• What is the impact of these changes? The same agent can immediately run again after closing an unstarted or partially consumed response. The regression tests also cover full consumption, workflow exceptions, cancellation, and repeated close of an old response while a newer run is active.
• What do you want reviewers to focus on? Cleanup must work before the first pull and must remain idempotent, so an old response cannot release a newer run's guard. The fix does not change the public API or persisted state.

Related Issue

Fixes #10164.

This submits the original patch I shared on November 7. On November 9, a maintainer marked the issue ready-for-implementation and assigned it to me. The branch preserves that original commit and incorporates the latest main.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution.

AI assistance details: implementation, regression tests, local validation, and this description were prepared with AI assistance. I personally ran the original minimal reproduction and observed the reported failure.

Contribution Checklist

• The core wheel and source distribution build successfully.
• All unit tests pass, and I have added new tests where possible — the relevant 249 functional workflow tests and 114 ResponseStream tests pass; the full repository suite was not run.
• The PR follows the Contribution Guidelines.
• This PR links to an issue marked ready-for-implementation by a maintainer.
• This is not a breaking change.

The two early-close regressions fail against the original implementation and pass with this fix. The original minimal reproduction now prints result: 2. Ruff lint and format, strict source Pyright, and test Pyright, mypy, Pyrefly, ty, and zuban checks pass for the changed files using the repository's pinned versions. Pyrefly reports three warnings in unchanged tests; the workflow tests emit the existing experimental-feature warning.