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 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.77%. Comparing base (f632f3f) to head (723a5cc).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_buffer.cc	66.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61914      +/-   ##
==========================================
- Coverage   91.70%   89.77%   -1.94%     
==========================================
  Files         337      674     +337     
  Lines      140104   204889   +64785     
  Branches    21758    39390   +17632     
==========================================
+ Hits       128484   183935   +55451     
- Misses      11398    13231    +1833     
- Partials      222     7723    +7501     

Files with missing lines	Coverage Δ
src/node_buffer.cc	67.77% <66.66%> (ø)

... and 458 files with indirect coverage changes

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[danielalanbates]

Thanks for the feedback, @Renegade334! I've pushed an update addressing your review comments. Please take another look when you get a chance.

[Renegade334]

Mostly good, modulo:

• Please format your initial commit message as per https://github.com/nodejs/node/blob/main/doc/contributing/pull-requests.md#commit-message-guidelines.
• Your initial commit does not have a valid user/email, so is not being attributed to you. Perhaps this was an agent configuration issue.
• Please address the linter comments by removing the custom assert message arguments to strictEqual.