[argparse] Allow string actions in add_subparsers

[nightcityblade]

Fixes #16309

The runtime accepts registered action names as strings in ArgumentParser.add_subparsers, just as add_argument does. This widens both overloads to accept str | type[Action] and adds a regression test using the built-in parsers registration.

Tests:

• pre-commit on both changed files: passed
• mypy stub checks on Python 3.10 through 3.15: passed
• stdlib regression tests on Python 3.10, 3.14, and 3.15: passed
• targeted Pyright, ty, and pyrefly checks: passed
• targeted stubtest for argparse with the repository allowlist: passed

[github-actions]

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

[donbarbos]

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.