Add optional gettext argument validation to Python extraction - #1460

Python message extraction currently accepts invalid gettext calls without diagnostics, such as gettext("hello", "extra") or ngettext("one", "many").

This adds an opt-in validate_gettext option that warns about incorrect argument counts and names using the standard library gettext signatures. Warnings include the source filename and line number.

Validation is disabled by default to support custom wrappers with different signatures. Calls using argument unpacking are skipped, and extraction results are preserved. Usage and limitations are documented.

Validation:

• All 513 tests in tests/messages pass.
• Applicable pre-commit checks pass.
• CLI extraction reports warnings for all three examples in Better argument validation for python *gettext functions #1002 and successfully writes the POT file.

Refs #1002