Handle checksum key error

[DavidMStraub]

No description provided.

[Copilot]

Pull request overview

This PR fixes an error path in media thumbnail/tile caching where a missing media handle could trigger failures during cache key computation (which flask_caching may log/swallow), ensuring the request reaches the client as a normal 404.

Changes:

• Move media lookup out of cache-key functions into an unless predicate that skips caching when the media handle does not exist.
• Update thumbnail/tile cache-key generation to rely on g.cached_media populated by the predicate.
• Add a regression test asserting missing-thumbnail requests return 404 without flask_caching error logs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/test_endpoints/test_file.py	Adds a regression test ensuring unknown thumbnail handles return a clean 404 without cache-backend error logs.
gramps_webapi/api/cache.py	Refactors thumbnail/tile caching to prefetch media in unless and avoid aborting during cache-key computation.

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