Remove validatePathInLibrary function and its fallback logic in
resolveRelativePath. The library matcher should always find the correct
library, including the playlist's own library. If this fails, we now
return an invalid resolution instead of attempting a fallback validation.
This simplifies the code by removing redundant validation logic that
was masking test setup issues. Also fixes test mock configuration to
properly set up library paths that match folder LibraryPath values.
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