test(filesdrop): fix duplicate scenario titles, indentation, and add clarifying comments

[joshtrichards]

Summary

Improves readability and correctness of filesdrop.feature without changing any test logic at this time.

• Rename two pairs of duplicate scenario titles (Put same file multiple times and invalid nickname with forbidden characters) so test reports are unambiguous
• Fix mixed tab/space indentation throughout the file
• Add a feature-level comment explaining the shareType 3 vs 4 split
• Add inline comments explaining the deduplication priority rules in the folder-vs-file conflict scenarios
• Add comments clarifying the 405 vs 404 distinction in the "prevents GET" scenario
• Document why dot-prefixed and .htaccess nicknames are blocked

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