refactor(files_sharing): clarify failed shared-storage cache handling

[joshtrichards]

• Resolves: #

Summary

Failed shared storage uses FailedCache’s synthetic root entry and is denied through normal permission checks.

This change acknowledges that ICache::get() can return false, caches that result, and makes isValid() explicitly deny access in that case. This prevents a cache miss from becoming a method call on false, while documenting why the normal FailedStorage path remains safe.

Changes:

• Make source-root cache-entry handling explicit when a cache lookup returns false.
• Reuse the cache installed during failed storage initialization.
• Replace the missing-source test setup with a deterministic mocked getById() miss.
• Verify missing-source shares are inaccessible without errors.

Depends on/related to: PR #62484

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