ffi: fix handle leak in fast buffer arguments

[trivikr]

Fixes: #66866

node_ffi_fast_buffer_data() runs inside a V8 Fast API call, where no HandleScope is open. It calls ArrayBufferView::Buffer(), which created a Local in the caller's scope on every optimized call. Over 2e7 calls, RSS grew from 50 MB to 675 MB.

This opens one HandleScope for the whole helper, matching Node's other fast callbacks. The new test uses v8.queryObjects() to check that optimized calls don't keep backing stores alive.

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Assisted-by: claude:opus-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

❌ Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.50%. Comparing base (56da329) to head (7cf5836).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ffi/fast.cc	22.22%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66867      +/-   ##
==========================================
- Coverage   90.51%   90.50%   -0.02%     
==========================================
  Files         797      797              
  Lines      278221   278213       -8     
  Branches    53463    53458       -5     
==========================================
- Hits       251844   251790      -54     
- Misses      16763    16799      +36     
- Partials     9614     9624      +10     

Files with missing lines	Coverage Δ
src/ffi/fast.cc	77.20% <22.22%> (+7.25%)	⬆️

... and 35 files with indirect coverage changes

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