fix: persist comments on reload in collab mode

[palmer-cl]

• Fixed collaboration refresh issues for comments and document content.

Summary

• Added comment hydration from Yjs on init/rejoin so sidebar comments auto-populate without
  manual comments injection.
• Ensured collaboration comment sync initializes once and re-hydrates safely when the comments store
  becomes available.
• Prevented duplicate comment sync listeners/observers during repeated init paths.

Tests

• Added focused collaboration tests for comment population:
  • Hydration from ydoc (including importedId normalization + dedupe).
  • Re-init behavior that re-hydrates store without duplicating listeners.

[linear]

SD-1260 Collaboration - comment load failure on page refresh

[palmer-cl]

@harbournick @caio-pizzol - let me know what the best way is to test this. I tested in multiple tabs, but Im not sure if theres a more robust way to test this PR?