Python: handle concurrent FileCheckpointStorage deletion

[fzfzzfzzzfzzzz]

Motivation & Context

FileCheckpointStorage.delete() checked for existence before unlinking the checkpoint file. Concurrent callers could both observe the file, after which one caller removed it and the other leaked an undocumented FileNotFoundError instead of returning False as required by the storage contract.

Description & Review Guide

• What are the major changes? The file storage now attempts unlink() directly and treats FileNotFoundError as a missing checkpoint. A deterministic regression test synchronizes the previous existence checks to reproduce the race.
• What is the impact of these changes? Concurrent deletes return one True and one False; other filesystem failures continue to propagate unchanged.
• What do you want reviewers to focus on? Please review the exception boundary, specifically that only FileNotFoundError is converted to False.

Related Issue

Fixes #8255

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 a language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

🟢 Approval recommended

The implementation is narrowly scoped, preserves other filesystem errors, and includes focused regression coverage.

Pull request overview

Makes file checkpoint deletion atomic under concurrent calls while preserving the storage contract.

Changes:

• Converts only FileNotFoundError from unlink() into False.
• Adds a deterministic concurrent-deletion regression test.

File summaries

File	Description
python/packages/core/agent_framework/_workflows/_checkpoint.py	Removes the check-then-delete race.
python/packages/core/tests/workflow/test_checkpoint.py	Verifies concurrent deletes return one True and one False.

Review details

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

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