fix: accept __index__ objects for unsigned integers without conversion so enum == numpy int works

[quinncheong]

Description

Closes #6192. Related: #5895.

Since #5887, py::enum_ comparison against a scalar uses typed overloads: (Type, Scalar) and then a catch-all (Type, const object &) that returns false. For an enum with an unsigned underlying type, the integer caster rejects __index__ objects (numpy integers among them) in the no-convert pass, because PyLong_AsUnsignedLong[Long] does not call __index__. The catch-all then matches in the same pass, so member == np.int32(3) is False while np.int32(3) == member is True. Signed underlying types are unaffected because PyLong_AsLong does call __index__.

The caster already calls PyNumber_Index explicitly on PyPy for the same reason. This change runs that path on CPython too when the target type is unsigned. The new test fails on master for the uint32_t enum and passes with the fix; the int32_t enum is the control.

Testing

Built pybind11_tests by hand from tests/pybind11_tests.cpp, tests/test_enum.cpp and tests/test_builtin_casters.cpp (g++ 11.5, -std=c++17, CPython 3.11, numpy 2.4) and ran pytest test_enum.py test_builtin_casters.py: on master the new test fails for the uint32_t enum (1 failed, 37 passed); with this change all 38 pass. I have not run the full suite, prek -a, PyPy, GraalPy or MSVC locally, so I am relying on CI for those.

Suggested changelog entry:

• Fixed enum equality against numpy integers and other __index__ objects when the enum's underlying type is unsigned (regression in 3.1.0); unsigned integer arguments now accept __index__ objects without implicit conversion, like signed ones.

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This PR was prepared with an AI coding agent (Claude Code); I reproduced the bug and ran the tests listed above before posting.

[quinncheong]

Pushed 2fa7d31 for the Clang-Tidy failure (readability-braces-around-statements on the if (!convert) return false; that used to be PyPy-only and now compiles on CPython too).

The other red jobs on the first run were not test failures: 21 were cancelled with "The job was not acquired by Runner of type hosted even after multiple attempts", and one Windows job lost contact with its runner. The test jobs that did run passed, across ubuntu, macos and windows at C++11 through C++23, including the free-threaded builds.

Prepared with an AI coding agent (Claude Code), checked by me.