ffi: preserve strings during reentrant calls

[trivikr]

Fixes: #64550

Temporary buffers used to convert JavaScript strings for Fast FFI calls were
cached per library and argument index. A reentrant callback could make another
FFI call using the same argument index, overwriting or replacing a buffer still
in use by the outer native call.

This changes the cache to be scoped by FFI wrapper and active call depth. Each
reentrant invocation therefore receives separate string storage while buffers
remain reusable across non-overlapping calls.

---

Assisted-by: openai:gpt-5.6-sol

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

❌ Patch coverage is 60.93750% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.24%. Comparing base (608112a) to head (fe9f7d0).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/ffi/fast-api.js	60.93%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64551      +/-   ##
==========================================
+ Coverage   90.21%   90.24%   +0.02%     
==========================================
  Files         739      739              
  Lines      241650   241737      +87     
  Branches    45542    45560      +18     
==========================================
+ Hits       218013   218147     +134     
+ Misses      15155    15119      -36     
+ Partials     8482     8471      -11     

Files with missing lines	Coverage Δ
lib/internal/ffi/fast-api.js	81.26% <60.93%> (-5.33%)	⬇️

... and 41 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74862/