fix: propagate errors from asset modification date lookup

[julianschiavo]

• Change modificationDate(for:) from silently returning .distantPast on failure to throwing, so callers surface the real error instead of mis-sorting assets
• Update the sort closure in locateDatabaseDirectoryURL() to propagate the throwing call
• Add regression test verifying modificationDate throws for a nonexistent URL

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Note

Low Risk
Low risk: small change to error propagation during asset selection plus a regression test; behavior only changes when modification date lookup fails (now throws instead of silently sorting as oldest).

Overview
Fixes asset selection error handling by making DocumentationAssetLocator.modificationDate(for:) throw on lookup failures instead of defaulting to .distantPast, and updating the candidate sort in locateDatabaseDirectoryURL() to propagate that error.

Adds a test ensuring a nonexistent asset URL causes modificationDate(for:) to throw, preventing silent mis-sorting of assets.

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