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 66.17647% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.13%. Comparing base (0992f6b) to head (414f602).

Files with missing lines	Patch %	Lines
lib/internal/ffi/fast-api.js	66.17%	22 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64551      +/-   ##
==========================================
- Coverage   90.14%   90.13%   -0.01%     
==========================================
  Files         741      741              
  Lines      242112   242153      +41     
  Branches    45605    45600       -5     
==========================================
+ Hits       218244   218266      +22     
- Misses      15360    15378      +18     
- Partials     8508     8509       +1     

Files with missing lines	Coverage Δ
lib/internal/ffi/fast-api.js	84.38% <66.17%> (-2.97%)	⬇️

... and 26 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74996/

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75038/