repro: preserve successful stage gitignore entries

[aryansk]

Fixes #11091

When a pipeline stage completes, keep its generated .gitignore entries committed to the reproduction result. If a later stage fails, only ignore entries created by the failing operation should be rolled back.

• ❗ I have followed the Contributing to DVC checklist.
• 📖 This PR does not require documentation updates.

Validation:

• PATH="$PWD/.venv/bin:$PATH" .venv/bin/pytest -q tests/func/repro/test_repro.py
• PATH="$PWD/.venv/bin:$PATH" .venv/bin/pytest -q tests/unit/repo/test_reproduce.py tests/unit/repo/test_scm_context.py
• .venv/bin/mypy dvc/repo/reproduce.py dvc/repo/scm_context.py
• ruff check and ruff format --check on changed files.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.98%. Comparing base (2431ec6) to head (f20b57f).
⚠️ Report is 213 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11094      +/-   ##
==========================================
+ Coverage   90.68%   90.98%   +0.30%     
==========================================
  Files         504      505       +1     
  Lines       39795    41146    +1351     
  Branches     3141     3263     +122     
==========================================
+ Hits        36087    37436    +1349     
- Misses       3042     3071      +29     
+ Partials      666      639      -27     

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