feat(gc): allow num with all-branches

[shcheklein]

dvc gc now supports using -n/--num with --all-branches, so garbage collection can preserve the last N commits from each branch tip, including when --projects repos share the same cache. This keeps the existing shared-cache union behavior and extends revision expansion rather than adding project-specific logic.

Example: dvc gc --all-branches --num 2 --projects ../repo-a ../repo-b --force

Docs will be updated when / if this is approved first.

---

• ❗ I have followed the Contributing to DVC checklist.

• ⌛ 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 91.42857% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.98%. Comparing base (2431ec6) to head (38bd8f3).
⚠️ Report is 194 commits behind head on main.

Files with missing lines	Patch %	Lines
dvc/commands/gc.py	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11013      +/-   ##
==========================================
+ Coverage   90.68%   90.98%   +0.29%     
==========================================
  Files         504      505       +1     
  Lines       39795    41132    +1337     
  Branches     3141     3261     +120     
==========================================
+ Hits        36087    37422    +1335     
- Misses       3042     3071      +29     
+ Partials      666      639      -27     

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[shcheklein]

@skshetry test failures are related to tabulate version bump it seems, not touching it here.