src: copy-construct data when cloning threadsafe COW

[Archkon]

Initialize the cloned data directly while holding the source read lock, so payloads do not need to be default-constructible or copy-assignable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.16%. Comparing base (c8e2a82) to head (773b304).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64859   +/-   ##
=======================================
  Coverage   90.16%   90.16%           
=======================================
  Files         746      746           
  Lines      242760   242763    +3     
  Branches    45765    45769    +4     
=======================================
+ Hits       218875   218879    +4     
- Misses      15375    15389   +14     
+ Partials     8510     8495   -15     

Files with missing lines	Coverage Δ
src/node_threadsafe_cow-inl.h	95.00% <100.00%> (+63.42%)	⬆️

... and 35 files with indirect coverage changes

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[Hardanish-Singh]

Optional test improvement

The existing assertion proves cloning succeeded, but the test could additionally demonstrate COW independence:

TEST(ThreadsafeCopyOnWriteTest, CloneCopyConstructsData) {
  const ThreadsafeCopyOnWrite<CopyConstructOnly> original(42);
  auto copy = original;

  copy.write()->value = 43;

  EXPECT_EQ(original.read()->value, 42);
  EXPECT_EQ(copy.read()->value, 43);
}