[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, so its regression cases are version-gated.

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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[github-actions]

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