fix(files_external/S3): normalize paths in touch()

[joshtrichards]

• Resolves: #

Summary

• Normalizes the incoming path up front, ensuring consistent key derivation and cache invalidation for edge-case paths (leading/trailing slashes, root-like values).
  • The existence check called here already does this internally already, but others (i.e. cleanKey()) do not. Doing it upfront is more consistent with the rest of the class and avoids weird Key values.
• Clarifies the intended behavior via comments with regard to mtime handling at this layer:
  • For existing objects we return false to let View::touch() emulate mtime updates in our filecache without performing a remote S3 operation
  • For create-if-missing the resulting cached mtime reflects S3’s LastModified rather than any caller-provided $mtime (since custom lastmodified metadata isn’t currently used even though we store it here).
• Some minor changes to improve code clarity

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