Python: handle concurrent FileSystemAgentFileStore deletion

[WhaleTech (ryo-whaletech)]

Motivation & Context

AgentFileStore.delete() returns True when deletion occurs and False when the file does not exist. Separate FileSystemAgentFileStore instances that share a root can race on the same filesystem entry and violate that result contract.

Description & Review Guide

• What are the major changes? Add a single process-local deletion lock around the existing file check and deletion, map only FileNotFoundError during deletion to False, and add deterministic cross-instance regressions for same-path and case-alias deletion.
• What is the impact of these changes? Concurrent deletes of the same filesystem entry within one process, with no intervening write, produce one deletion result and one missing-file result. Existing directory/non-file semantics, path and symlink safety, and propagation of unrelated filesystem errors remain unchanged. This does not provide cross-process serialization or delete/write atomicity.
• What do you want reviewers to focus on? The process-local locking scope, preservation of existing non-file behavior, narrow exception handling, and the regressions' ability to fail when synchronization is removed.

Related Issue

Fixes #8456

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

🟢 Approval recommended

The implementation preserves existing behavior while correctly covering the reported race with a focused regression test.

Pull request overview

Adds process-local synchronization to preserve FileSystemAgentFileStore.delete() result semantics during concurrent same-path deletion.

Changes:

• Adds bounded striped locks around file checks and deletion.
• Maps deletion-time FileNotFoundError to False.
• Adds a deterministic cross-instance concurrency regression test.

File summaries

File	Description
python/packages/core/agent_framework/_harness/_file_access.py	Serializes same-stripe deletions and handles deletion races.
python/packages/core/tests/core/test_harness_file_access.py	Tests concurrent deletion across store instances.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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