[ibrahimmaq14]

Fixes #11400.

Change the stored _initializer attributes from Callable[..., None] | None to Callable[..., object] | None to match the constructor annotations, accepting value-returning callbacks while retaining checks for non-callables and mismatched constructor initargs.

CPython stores the callable unchanged and ignores its result (process executor, thread executor through 3.13); the thread attribute was removed in 3.14.

Verified locally:

• python tests/runtests.py stdlib/concurrent: all checks passed, including pre-commit, structure, mypy, pyright, ty, pyrefly, stubtest, and regression suites.
• Full stdlib mypy regression suite and focused mypy/pyright cases: Python targets 3.10–3.15.
• Concurrent stubs: mypy targets 3.10–3.15; pyright, ty, and pyrefly targets 3.10 and 3.14.
• Runtime smoke tests on CPython 3.11.9 and 3.14.7; stubtest on 3.11.9 (899 modules) and 3.14.7 (the two changed modules).

Agent used: OpenAI Codex.

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[srittau]

Please remove the tests. In typeshed, we only add regression tests for functions and classes which are known to have caused complex problems in the past, or where stubs are difficult to get right. 100% test coverage for typeshed is neither necessary nor desirable, as it would lead to code duplication.

See tests/REGRESSION.md for more information.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉