perf(files_external/S3): slightly optimize memory in opendir

[joshtrichards]

• Resolves: #

Summary

• A small optimization from only carrying forward the key we care about into the final array.
• Also: slightly better variable naming and exception logging (rather than silently swallowing every S3Exception)

TODO

• A much bigger win (in terms of memory/CPU usage for larger directories) would be a rewind-safe iterator.

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