Improve invalid source type error

[Martonveghcode]

Summary

• Update the MarkItDown.convert() unsupported-source TypeError to list pathlib.Path, which is already accepted by the API.
• Add a focused regression test that checks the error message lists the supported source types.

Validation

• python -m pytest packages/markitdown/tests/test_module_misc.py::test_invalid_source_type_error_lists_supported_types packages/markitdown/tests/test_module_misc.py::test_exceptions
• python -m black --check packages/markitdown/src/markitdown/_markitdown.py packages/markitdown/tests/test_module_misc.py