ffi: keep FFI functions non-constructible

[trivikr]

Fixes: #65183

Use concise method functions for Fast API and shared-buffer wrappers, and create native fallback functions with ConstructorBehavior::kThrow, so FFI functions remain non-constructible on all invocation paths.

---

Assisted-by: codex:gpt-5.6-sol

[nodejs-github-bot]

Review requested:

• @nodejs/ffi

[codecov]

Codecov Report

❌ Patch coverage is 47.82609% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.33%. Comparing base (6fec5e8) to head (edd2a79).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/ffi-shared-buffer.js	33.33%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65184      +/-   ##
==========================================
+ Coverage   90.32%   90.33%   +0.01%     
==========================================
  Files         760      760              
  Lines      248523   248527       +4     
  Branches    46896    46887       -9     
==========================================
+ Hits       224469   224507      +38     
+ Misses      15465    15459       -6     
+ Partials     8589     8561      -28     

Files with missing lines	Coverage Δ
lib/internal/ffi/fast-api.js	94.80% <100.00%> (+0.02%)	⬆️
lib/internal/ffi-shared-buffer.js	55.57% <33.33%> (+0.16%)	⬆️

... and 29 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75753/

[trivikr]

Converted to draft since test-ffi-dynamic-library is failing in https://ci.nodejs.org/job/node-test-commit-aix/64326/

not ok 3 ffi/test-ffi-dynamic-library
  ---
  duration_ms: 800.27300
  severity: fail
  exitcode: 1
  stack: |-
    Test failure: 'FFI functions are not constructible'
    Location: test/ffi/test-ffi-dynamic-library.js:70:1
    AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:
    
    true !== false
    
        at TestContext.<anonymous> (/home/iojs/build/workspace/node-test-commit-aix/nodes/aix72-power9/test/ffi/test-ffi-dynamic-library.js:77:12)
        at Test.runInAsyncScope (node:async_hooks:227:14)
        at Test.run (node:internal/test_runner/test:1397:25)
        at Test.processPendingSubtests (node:internal/test_runner/test:969:18)
        at Test.postRun (node:internal/test_runner/test:1537:19)
        at Test.run (node:internal/test_runner/test:1462:12)
        at process.processTicksAndRejections (node:internal/process/task_queues:104:5)
        at async Test.processPendingSubtests (node:internal/test_runner/test:969:7) {
      generatedMessage: true,
      code: 'ERR_ASSERTION',
      actual: true,
      expected: false,
      operator: 'strictEqual',
      diff: 'simple'
    }
    
    (node:31457608) ExperimentalWarning: FFI is an experimental feature and might change at any time
    (Use `node --trace-warnings ...` to show where the warning was created)
  ...

[trivikr]

The AIX failure occurred because AIX uses neither of the JavaScript wrapper paths updated in the original commit.

AIX/PowerPC is excluded from the Fast API path because there is no PPC64 big-endian trampoline emitter. The shared-buffer path is also disabled on big-endian platforms. Consequently, AIX falls back to the generic native function created with v8::Function::New().

Function::New() defaults to ConstructorBehavior::kAllow, so the fallback function had its own prototype and remained constructible. Since it had no Fast API or shared-buffer metadata, it was returned without a JavaScript wrapper.

I amended the commit to create the generic fallback and shared-buffer slow-path functions with ConstructorBehavior::kThrow. The concise-method changes still ensure that JavaScript wrappers are non-constructible, while the C++ change covers native fallback paths such as AIX.