Let the borrowed() hook survive an overload that did not match

[julianspeith]

Summary

Fixes the macOS CI failure in runTest-python_binding_smoke that has been present on master since the runner's Homebrew pybind11 moved to 3.1.0.

Since pybind11 3.1.0, cpp_function::initialize runs the post-call hook of every attribute even when the overload's arguments did not load, handing it the try-next-overload sentinel ((PyObject*) 1) instead of an object. Before 3.1.0 a failed argument load returned before the hook. HAL's hal::borrowed() hook treated the sentinel as an object and segfaulted on the first binding whose first overload rejects the call. In the smoke test that is Netlist.create_module(name, parent, gates), whose four-argument overload is registered first.

The hook now returns early on a null or sentinel handle. That is the whole change, plus a changelog entry.

Verification

• Reproduced locally only after installing the runner's versions (Homebrew Python 3.14.7, pybind11 3.1.0): the smoke test segfaults at the same line with the same C stack as CI. Earlier attempts with a pip-installed pybind11 3.1.0 silently compiled against Homebrew's 3.0.1 headers in /opt/homebrew/include, which is why the crash looked environment-specific for so long.
• With this change the full suite passes, 39 of 39, on a Debug build against those versions.
• pybind11's own py::keep_alive<0, 1> crashes on the same input with 3.1.0 and not with 3.0.4 (minimal reproducer: two overloads with keep_alive<0, 1>, call with arguments matching only the second). This is an upstream regression and worth reporting to pybind11; the guard here is correct regardless.

Test plan

• macOS CI green, including runTest-python_binding_smoke
• Ubuntu jobs unchanged

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