fix(Files/Cache): align MoveFromCacheTrait fallback validation with Cache::moveFromCache

[joshtrichards]

Summary

Validate the source cache entry in MoveFromCacheTrait::moveFromCache() before attempting the fallback copy-then-delete move.

Why

The fallback path should fail consistently when the source entry does not exist, rather than relying on a less specific downstream failure. This also keeps the trait behavior aligned with Cache::moveFromCache().

Behavior change

Low risk:

• If the source path is missing, this moveFromCache() now throws a clearer RuntimeException immediately.
• This matches the validation already performed by Cache::moveFromCache().
• The observable behavior is unchanged for callers that already depended on an exception; only the failure location and message are improved.

TODO

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

An side: MoveFromCacheTrait currently appears to be an implementation detail of OC\Files\Cache\Cache rather than a reusable trait.

Since it is only used in production by Cache, I lean toward folding the fallback into the class itself to make ownership and behavior clearer.

I was initially confused about its use case, since no production code uses the trait other than Cache (which I wouldn’t have expected to need the fallback), but then I realized that the source caches that may use this fallback still ultimately flow through Cache, which is the one responsible for deciding whether to use the fallback or not.