src: stop leaking a CppHeap in CommonEnvironmentSetup

[codebytere]

CommonEnvironmentSetup's constructor created a v8::CppHeap for its Isolate::CreateParams up front, but on the non-snapshotting path it then calls NewIsolate(), which ignores params->cpp_heap and attaches either the heap from IsolateSettings or a fresh one. The heap created first was never attached to an isolate and never destroyed, so every CommonEnvironmentSetup::Create() / CreateFromSnapshot() leaked one CppHeap. cppgc's process-wide heap registry keeps a pointer to it, which is why LeakSanitizer does not report it; counting CppHeap constructions against destructions over two setups shows 4 against 2 before this change and 2 against 2 after.

The heap is now only created on the snapshotting path, where the params are handed to the SnapshotCreator and the heap does get attached.

Tests: existing test/embedding suite; no new test, since there is no public API to enumerate detached CppHeaps.

Refs: #55337

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Disclosure: the code and this description were written by Claude Code, directed and reviewed by @codebytere.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/76973/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (2befec5) to head (455f726).
⚠️ Report is 77 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65792      +/-   ##
==========================================
+ Coverage   89.99%   90.15%   +0.15%     
==========================================
  Files         757      769      +12     
  Lines      257739   261448    +3709     
  Branches    48881    49661     +780     
==========================================
+ Hits       231961   235715    +3754     
+ Misses      16861    16749     -112     
- Partials     8917     8984      +67     

Files with missing lines	Coverage Δ
src/api/embed_helpers.cc	75.00% <100.00%> (ø)

... and 101 files with indirect coverage changes

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