#40067 refactor: Remove unused `description` from attachments render

Proposed changes (including videos or screenshots)

Summary

• Remove descriptionMd field from MessageAttachmentBase type
• Remove descriptionMd parsing from BeforeSaveMarkdownParser and parseMessageTextToAstMarkdown
• Remove visible description rendering from all file attachment components (Image, Audio, Video, Generic)
• Remove descriptionMd fallbacks from copy/report message actions
• Clean up related tests

The descriptionMd field was a parsed markdown AST of the attachment description, rendered as text above file attachments. This is no longer needed as description is being repurposed exclusively for image alt text.

Test plan

• Upload an image — verify no description text renders above the image
• Upload audio/video/generic files — verify no regressions
• Copy and report message actions still work on file messages

Issue(s)

Steps to test or reproduce

Further comments

WA-17

Summary by CodeRabbit

• Changes
  • Attachment descriptions no longer display in audio, video, image, and generic file previews, exported PDFs, email notifications, transcripts, and exported attachment payloads.
  • Copy/report fallbacks, message editing, previews, thread titles, decrypted-message display, and normalized previews now prefer attachment titles or message text instead of descriptions.
  • Auto-translation and translation providers no longer send or derive from attachment descriptions.
  • Public typing: descriptionMd removed; tests updated.