src: delete cppgc tracking nodes during Realm cleanup

[Archkon]

Delete CppgcWrapperListNode instances after finalizing their wrappers to prevent native memory leaks when a Realm is destroyed while wrappers remain alive.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.26%. Comparing base (cd78d08) to head (bef1ff8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65000      +/-   ##
==========================================
- Coverage   90.28%   90.26%   -0.02%     
==========================================
  Files         762      762              
  Lines      247534   247537       +3     
  Branches    46684    46692       +8     
==========================================
- Hits       223479   223444      -35     
- Misses      15485    15537      +52     
+ Partials     8570     8556      -14     

Files with missing lines	Coverage Δ
src/cppgc_helpers.cc	77.77% <100.00%> (-13.89%)	⬇️

... and 27 files with indirect coverage changes

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