Update cppgc_helpers.cc

[gugu8intel-i9]

This PR optimizes CppgcWrapperList::PurgeEmpty() by replacing redundant iterator copies with in-place increments and removing unnecessary manual handle clearing. It also modernizes Cleanup() and MemoryInfo() using scoped variable initialization for improved readability and performance.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.18%. Comparing base (695f01f) to head (62ffbdc).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cppgc_helpers.cc	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65871      +/-   ##
==========================================
- Coverage   90.19%   90.18%   -0.02%     
==========================================
  Files         771      771              
  Lines      264628   264624       -4     
  Branches    50237    50238       +1     
==========================================
- Hits       238683   238642      -41     
- Misses      16953    16962       +9     
- Partials     8992     9020      +28     

Files with missing lines	Coverage Δ
src/cppgc_helpers.cc	75.00% <66.66%> (ø)

... and 24 files with indirect coverage changes

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[MikeMcC399]

See Commit message guidelines according to the Pull requests documentation.

Your previously submitted invalid PR #65360 was rejected partially for the same reason.

[MikeMcC399]

This PR is failing linting tests. See the comments in the tests for more detail.

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