Add disabled_converters support for built-ins

[txhno]

Closes #1665.

Adds a disabled_converters option to built-in registration and covers it with focused tests.

Validation:

• pytest tests/test_module_misc.py -k "disabled_converters or enable_builtins_with_disabled_converters"
• direct runtime check in the repo venv

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[VANDRANKI]

Thanks for implementing this. The approach is clean and addresses what the issue asked for.

maybe_register helper - good pattern. Keeps the registration block readable without repeating the guard on every line.

Class name strings - type(converter).__name__ not in disabled_converters is the right check. Callers do not need to import converter classes, which was the core ask.

Priority refactor - I checked: register_converter defaults to PRIORITY_SPECIFIC_FILE_FORMAT, so assigning it explicitly to RssConverter, WikipediaConverter, etc. only makes the implicit default visible. No behavioral change.

Tests - both paths covered: MarkItDown(disabled_converters=...) constructor and enable_builtins(disabled_converters=...) directly. The registered_converter_names helper is reusable.

LGTM.