src: fix heap value deduplication in embedder graph

[IlyasShabi]

This PR aims to fix an embedder graph bug introduced by #52295, where distinct V8 values could be treated as the same graph node.

It uses an explicit value lookup for deduplication and extends the internal test binding to expose V8 values for regression testing.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.14%. Comparing base (4bec191) to head (d62e941).
⚠️ Report is 112 commits behind head on main.

Files with missing lines	Patch %	Lines
src/heap_utils.cc	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64801      +/-   ##
==========================================
+ Coverage   90.13%   90.14%   +0.01%     
==========================================
  Files         741      744       +3     
  Lines      242106   242542     +436     
  Branches    45551    45692     +141     
==========================================
+ Hits       218211   218648     +437     
+ Misses      15430    15394      -36     
- Partials     8465     8500      +35     

Files with missing lines	Coverage Δ
src/heap_utils.cc	80.61% <86.66%> (+1.36%)	⬆️

... and 93 files with indirect coverage changes

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