feat: Save a checksum for documents and use it to detect conflicts

[benjaminfrueh]

📝 Summary

Text sometimes throws a DocumentSaveConflictException when a files mtime/etag is updated without changing the content.

Solution:
Compute and store a CRC32 checksum for each document. On saving or syncing, the checksum is updated, and assertNoOutsideConflict() uses it to detect real content changes, preventing false conflicts.

🏁 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

[max-nextcloud]

Thanks a lot for looking into this!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.52%. Comparing base (2545f19) to head (c4d8dfd).
⚠️ Report is 104 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7677      +/-   ##
==========================================
- Coverage   59.57%   59.52%   -0.06%     
==========================================
  Files         502      503       +1     
  Lines       39059    39118      +59     
  Branches     1129     1130       +1     
==========================================
+ Hits        23270    23285      +15     
- Misses      15681    15725      +44     
  Partials      108      108              

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[tobiasKaminsky]

Tested with Android Notes app and works 🎉

[tobiasKaminsky]

/backport to stable31

[mejo-]

/backport to stable32

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[juliusknorr]

@benjaminfrueh I found that it seems less likely but still sometimes happens on our instance that we see a conflict during saves.

I managed to see this if a save request happened to take longer (2-3s) and i meanwhile continued typing:

Maybe there is a way to reproduce that locally with some manual delay? I'm wondering if we can prevent throwing conflicts during times another php process is currently handling a save