Disable compiler stack switching in Python 3.14

[alexreinking]

Python 3.14 caches C stack limits in the thread state, which does not interact well with Halide's user-space compiler stack when warnings call back into Python. Stay on Python's stack by default for Python 3.14+, while preserving the existing Halide behavior when HL_COMPILER_STACK_SIZE is explicitly set.

Fixes #9167

Tested:

cmake --build build/macOS --target Halide_Python -j8
uv run --no-sync ctest --test-dir build/macOS -R python_correctness_realize_warnings -V
uv run --no-sync ctest --test-dir build/macOS -L python --output-on-failure

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9168   +/-   ##
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  Coverage        ?   69.35%           
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  Files           ?      254           
  Lines           ?    78216           
  Branches        ?    18712           
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  Hits            ?    54247           
  Misses          ?    18480           
  Partials        ?     5489           

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[alexreinking]

Failure appears intermittent / obviously unrelated (OpenCL, not in Python).