fix(agentserver): release Windows replay locks on process exit

[Shikhar Goel (sgoel2be24-cyber)]

Description

Fixes #48603.

On Windows, an abrupt process exit leaves the replay stream's marker file behind, so replacement processes cannot recover the stream. Replace marker-file ownership with a nonblocking msvcrt.locking byte lock, released by the OS when the descriptor closes or its process dies. POSIX locking is unchanged.

The sidecar stays on disk deliberately: existence no longer means ownership, and unlinking it could split contenders across different files. Cleanup closes the lock descriptor once; failed acquisition closes both descriptors and preserves non-contention I/O errors. No dependency or public API changes.

Tests cover real subprocess os._exit(86) recovery with replay/continued writes, rejection while another process is alive and recovery after killing it, Windows stale-sidecar recovery, and mocked Windows error cleanup on any platform. The mocked tests are not evidence of native Windows execution. Four error-path cases fail on unchanged upstream and pass with the fix.

Developed with assistance from OpenAI Codex.

All SDK Contribution checklist:

• The pull request does not introduce breaking changes.
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message.

Testing Guidelines

• Pull request includes test coverage for the included changes.

Local validation on macOS / Python 3.12:

• Core suite: 978 passed, 8 skipped.
• Combined core/responses suite (including subprocess resilience E2E): 2,503 passed, 21 skipped; six async tests initially failed to execute because the root invocation omitted responses' asyncio_mode=auto. Rerunning their entire test_sse_writer.py with the responses pyproject.toml gave 11 passed, resolving all six invocation failures.
• Streaming suite: 122 passed, 1 skipped; file-backed suite: 33 passed, 1 skipped.
• Core Pylint 4.0.4 with Azure plugin 0.5.7: 10.00/10; MyPy 1.19.1: 53 files clean; changed-module Pyright 1.1.407: 0 errors/warnings; Black and git diff --check: clean.

Commands from the repository root (with the local core/responses packages and dev dependencies installed):

python -m pytest sdk/agentserver/azure-ai-agentserver-core/tests -q
python -m pytest sdk/agentserver/azure-ai-agentserver-core/tests sdk/agentserver/azure-ai-agentserver-responses/tests -q --import-mode=importlib
python -m pytest -c sdk/agentserver/azure-ai-agentserver-responses/pyproject.toml sdk/agentserver/azure-ai-agentserver-responses/tests/unit/test_sse_writer.py -q

Native Windows execution is pending: this development machine is macOS. The existing agentserver CI matrix includes Windows Server 2022/Python 3.12; no pipeline changes are included. This PR remains a draft pending native Windows validation and review, per repository agent guidance.

Full-core Pyright also reports an existing _tracing.py:851 TracerProvider.add_span_processor error with the installed telemetry dependencies. The identical error reproduces on unchanged upstream ae81bbf1c9ed59e6f06c48d7aa0e815d8c2396df; the changed streaming module passes with zero errors/warnings.

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[github-actions]

Thank you for your contribution Shikhar Goel (@sgoel2be24-cyber)! We will review the pull request and get back to you soon.