Treat a vanished entry as a miss in FileTreeStore.get_entry

[jonaseck2]

Closes #366.

get_entry checks that the file exists and then opens it. If something deletes the entry in between, read_file raises FileNotFoundError and it reaches the caller.

This returns None instead. That is what get already returns when the file is gone at the exists() check, and what delete_entry does in the same class.

The test removes the file inside the exists() call, following test_delete_returns_false_when_file_disappears_before_unlink, so the interleaving is deterministic rather than timing-dependent.

Checks on Python 3.12: ruff format and ruff check clean, basedpyright 0 errors, and the two filetree race tests pass. Without the change in store.py the new test fails with FileNotFoundError.

Context for why this matters in practice is in #366: we hit it on FastMCP's OAuthProxy, where one-time-use refresh tokens are deleted as part of normal rotation, and the exception turned a cache miss the caller handles into an HTTP 500.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Reviewing files that changed from the base of the PR and between 5929cff and 2490ce9.

📒 Files selected for processing (2)

• src/key_value/aio/stores/filetree/store.py
• tests/stores/filetree/test_filetree.py

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Walkthrough

DiskCollectionInfo.get_entry now catches FileNotFoundError when a file is removed after the existence check. It returns None, matching the behavior for an entry that is already missing. A regression test reproduces this race and verifies that FileTreeStore.get returns None instead of propagating the exception.

Merge Risk: ⚪ Minimal · up to 2490c

This localized change treats an entry deleted during lookup as a cache miss instead of raising an error, with targeted tests and clean checks reported; no actionable merge-blocking risk remains after normal review.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The change catches FileNotFoundError during the read and returns None, with a regression test covering deletion after exists().
Out of Scope Changes check	✅ Passed	The changes are limited to the requested race-condition fix and its regression test.

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[cubic-dev-ai]

No issues found across 2 files

<sub>Auto-approved: Focused bug fix: catches FileNotFoundError between exists() and read to return None, matching existing get/delete_entry miss semantics, with a deterministic race test. No design or operational tradeoff requires human judgment.

Re-trigger cubic</sub>