ffi: throw on missing memory helper arguments

[soulee-dev]

ffi.getInt8() through ffi.getFloat64(), ffi.setInt8() through ffi.setFloat64(), ffi.toBuffer() and ffi.toArrayBuffer() return undefined instead of throwing when a required argument is omitted, so a call that read or wrote nothing cannot be told apart from one that read a zero byte.

GetValidatedPointerAddress() and GetValidatedSize() already reject the same argument when it is passed explicitly as undefined. The args.Length() test in front of them short-circuits the call and returns Nothing without scheduling an exception. These six are the only tests in src/ where args.Length() can skip a call that throws; the only other Length() tests that guard a call at all guard Buffer::HasInstance(), which cannot throw.

$ # before
$ node --experimental-ffi -p "require('node:ffi').getUint8()"
undefined

$ # after
$ node --experimental-ffi -p "require('node:ffi').getUint8()"
TypeError: The pointer must be a bigint

Drop those tests. FunctionCallbackInfo::operator[] returns Undefined for an out-of-range index, which is exactly the value these validators reject, so each missing argument now produces the error its explicit undefined counterpart produces. The tests that guard an explicit throw, such as the ones in ToString() and GetRawPointer(), are left alone.

ExportBytes() carried the same two tests. They are unreachable through the public API because exportBytes is not exported and its three callers all validate len in JavaScript first, but they are the same shape.

No documentation change is needed: doc/api/ffi.md already lists these arguments as required.

Fixes: #65499
Refs: #62072
Refs: #62858

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (a48e33f) to head (8926b4d).
⚠️ Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ffi/data.cc	66.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #65500    +/-   ##
========================================
  Coverage   90.12%   90.12%            
========================================
  Files         751      751            
  Lines      252425   252589   +164     
  Branches    47471    47535    +64     
========================================
+ Hits       227504   227655   +151     
- Misses      16216    16239    +23     
+ Partials     8705     8695    -10     

Files with missing lines	Coverage Δ
src/ffi/data.cc	77.55% <66.66%> (+1.83%)	⬆️

... and 61 files with indirect coverage changes

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