[xiehuanyi]

A valid callback can evaluate as false, for example an initially empty callable list used to collect expiration notifications. WeakFunctionProxy passed this callback directly to weakref.ref instead of wrapping it, so it received a weak reference instead of the public proxy. For a bound method, expiration of both the instance and function could also invoke it twice.

Treat only None as the absence of a callback. False-valued callable callbacks now use the same proxy forwarding and once-only expiration handling as other callbacks. Add regressions for free functions and bound methods, plus an unreleased change note.

Validation on Linux:

• Both new cases fail before the fix with the C extension enabled and disabled, and pass after it.
• The complete just test matrix passes: 27 full-suite runs over Python 3.9–3.15, including 3.14t and 3.15t, each with pure-Python installation, C extension enabled, and C extension disabled at runtime. The recipe's pinned mypy pair tests run in this matrix.
• Final targeted weak-proxy suites pass in both implementations (16 tests each).
• git diff --check passes. Black's remaining test-file findings are also present on develop; the added code is formatted without unrelated changes.

AI assistance was used to prepare the implementation and tests. The commands above were executed locally. Remote CI remains subject to the repository's normal approval workflow.