gh-149196: Hide executor implementation details from ABI tools

[Taeknology]

Summary

Hide the concrete executor array and executor object layouts from ABI analysis
while preserving their existing runtime representation and behavior.

_PyExecutorArray was concretely defined in the non-limited code object header.
This made the private _PyExecutorObject layout ABI-reachable through
PyCodeObject.co_executors.

Moving the array representation alone was not sufficient. A controlled ABI
probe showed that _PyExecutorObject remained reachable through _PyExitData
and exported executor and JIT interfaces intended for internal use.

This change therefore:

• leaves only an incomplete declaration of _PyExecutorArray in
  Include/cpython/code.h;
• defines a separate concrete array representation in the internal optimizer
  header;
• introduces _PyExecutorHandle at exported executor and JIT interface
  boundaries;
• keeps _PyExecutorObject and the concrete executor array representation
  internal;
• preserves the existing field layout, allocation behavior, exported symbol
  names, and symbol visibility;
• does not add a type-specific libabigail suppression.

The apparent 12-file footprint includes three files generated from
Python/bytecodes.c. The remaining consumer changes are mechanical adaptations
required by the opaque array and handle types.

ABI verification

I used a controlled probe that adds a field to _PyExecutorObject in a separate
temporary source tree.

Before this change, suppression-enabled abidiff reported the probe as an ABI
difference and exited with status 4.

After this change, suppression-enabled abidiff exited with status 0 in two
runs for each of the following configurations:

• a standard shared debug build;
• an experimental JIT-interpreter shared debug build.

Both direct library comparisons and ABI XML comparisons passed. The exported
executor and JIT symbols remained present.

Tests

• Standard debug shared build
• Standard --fast-ci: 48,060 tests
• Free-threaded debug shared build
• Free-threaded --fast-ci: 51,526 tests
• Repeated executor, opcode, monitoring, tracing, C API, and code-object tests
• Repeated focused experimental JIT-interpreter tests with PYTHON_JIT=1
• Reference-leak checks for the core affected tests
• Checks that generated case files were up to date after regeneration
• make patchcheck
• git diff --check

This PR includes no documentation changes or NEWS entry because it changes only
the visibility of private implementation details, without changing
user-visible behavior or any supported public API.

Closes #149196

• Issue: _PyExecutorObject is exposed to the abidiff tool due to indirect use in PyCodeObject #149196