fix(collaboration): override addProseMirrorPlugins to prevent default caret user

[benjaminfrueh]

📝 Summary

Fixes the collaborator cursor showing a id (e.g. User: 123) instead of the actual name in stable35.

What happened

Tiptap sets its own default user when the caret feature is setup. Since a recent tiptap update (>= 3.24.0), which we use now with stable35, this happens later and ends up overwriting the name we set.

Fix

Override tiptap's addProseMirrorPlugins in our caret extension so it no longer sets a default user, leaving our user data as the only source.

Related tiptap commit which introduced this incompatibility:
ueberdosis/tiptap@976b8e3

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI tools
• The AI-generated content was reviewed, comprehended and tested by a human

[benjaminfrueh]

/backport to stable35

[max-nextcloud]

Code changes look good.