Call function signature hooks when checking decorators

[Cherith1222]

Summary

Decorator calls never reached get_function_signature_hook, so a plugin could not adjust that signature the way it can for a normal call. Checking a decorator now invokes the hook.

Fixes #21904.

Test Plan

On Windows, Python from this checkout's .venv, PYTEST_DISABLE_PLUGIN_AUTOLOAD=1, Resource Sentinel MEDIUM (1 CPU, 2.5–3 GiB):

• Before the change, the new test failed. After it, the new test and two existing plugin tests each passed.
• python -m pytest -o addopts= -p no:cacheprovider mypy/test/testcheck.py::TypeCheckSuite::check-custom-plugin.test -q → 65 passed.
• The same prefix with mypy/test/testcheck.py -k decorator → 112 passed, 8205 deselected.
• check-functions.test, check-functools.test, check-singledispatch.test, check-plugin-attrs.test, check-dataclasses.test, and check-dataclass-transform.test → 658 passed, 2 xfailed (the two existing -xfail cases in check-singledispatch.test).

Not run: the full mypy suite, mypy_primer, the daemon / fine-grained mode, and mypyc.

AI assistance

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[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy-stubs (https://github.com/scipy/scipy-stubs)
- tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports