[argparse] Allow string actions in add_subparsers - #16318

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