Fix #55422: Buffer.concat and Buffer.copy silently produce invalid r

[danielalanbates]

Fixes: #55422

Summary

This PR fixes: Buffer.concat and Buffer.copy silently produce invalid results when the operation involves indices equal or greater than 2^32

Changes

src/node_buffer.cc                      | 24 ++++++++++++------
 test/parallel/test-buffer-copy-large.js | 44 +++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+), 8 deletions(-)

Testing

Please review the changes carefully. The fix was verified against the existing test suite.

---

This PR was created with the assistance of Claude Sonnet 4.6 by Anthropic | effort: high. Happy to make any adjustments!

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.75%. Comparing base (f632f3f) to head (9313bcf).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_buffer.cc	72.72%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61914      +/-   ##
==========================================
- Coverage   91.70%   89.75%   -1.95%     
==========================================
  Files         337      674     +337     
  Lines      140104   204892   +64788     
  Branches    21758    39380   +17622     
==========================================
+ Hits       128484   183911   +55427     
- Misses      11398    13240    +1842     
- Partials      222     7741    +7519     

Files with missing lines	Coverage Δ
src/node_buffer.cc	67.87% <72.72%> (ø)

... and 455 files with indirect coverage changes

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