fix: type *devices/*device_uids as str, not List[str]

[proscar87]

Summary

Closes #2828. For a variadic parameter, the annotation types each individual argument, not the collection they get gathered into at the call site -- *devices: str means "each positional arg is a str", matching how these functions are actually called (set_output_devices(*device_ids) with a list of individual device ID strings unpacked via *) and how they're implemented internally (each element treated as one string, never a list, at every call site in this codebase).

Same bug in four places, all fixed the same way:

• pyatv/interface.py -- the three abstract declarations (add_output_devices, remove_output_devices, set_output_devices)
• pyatv/core/facade.py -- the facade relay, which just forwards *devices unchanged
• pyatv/protocols/mrp/__init__.py -- the MRP protocol implementation
• pyatv/protocols/mrp/messages.py -- the underlying protobuf message builders (add_output_devices/remove_output_devices/set_output_devices), which iterate device_uids and append each one individually to a repeated protobuf field -- one more instance of the same mistake the issue didn't call out, found while checking every call site

List became an unused import in messages.py after the fix; removed.

Testing

This is a pure static-typing annotation fix with no runtime behavior change, so verification is via mypy, not pytest assertions:

• Reproduced the exact error from the issue in isolation first: a minimal *devices: List[str] class definition, called the way the issue describes, gives mypy's exact reported message (incompatible type "*list[str]"; expected "list[str]"). The same pattern with *devices: str type-checks clean.
• Ran mypy (the project's own pyatv --ignore-missing-imports --follow-imports=skip invocation from chickn.yaml) before and after on the full pyatv package: 14 pre-existing, unrelated errors in both runs (in dns.py, http.py, metadata.py, pairing.py, scan.py, one line of mrp/__init__.py unrelated to output devices, and __init__.py) -- none of them ever mention output_devices, and the count and file list are identical before/after this change.
• Full test suite: 1245 passed, 1 pre-existing unrelated failure (test_scan_with_zeroconf_complete_and_device_info_specific_host_matching, an IndexError that reproduces identically on unmodified main -- confirmed via git stash before/after), 6 skipped.
• black --check, flake8, and pylint (10.00/10) all clean on the four changed files.

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