[jameshilliard]

Setuptools honors SETUPTOOLS_EXT_SUFFIX when generating extension filenames, but setuptools-rust always removes the length of the sysconfig EXT_SUFFIX when replacing it with a stable-ABI suffix. An override with a different length can truncate the module name or leave part of the old suffix in the filename.

Use the nonempty environment override to determine the suffix being replaced, falling back to sysconfig for unset or empty overrides, matching setuptools.

Add regression tests covering unset, empty, shorter, and longer overrides with unqualified and platform-qualified ABI3/ABI3T suffixes, .pyd, and platforms without a stable-ABI suffix. Add a changelog entry.

Testing

• Project tests: 36 passed with setuptools 84.2.1.
• Ruff lint and formatting checks, and mypy passed.
• Cross-built AArch64 limited-API wheels with short and long suffix overrides. Both contained the correct rust.abi3.so filename and cp39-abi3-linux_aarch64 wheel tags, and imported and executed successfully under QEMU with Python 3.14.