[rejected AI] privatize convert_type

[mahesh-desu]

Closes #3847.

Renames \click.types.convert_type\ to _convert_type; the old name remains as a deprecated shim emitting \DeprecationWarning\ (removed in Click 9.0). Internal callers in \ ypes.py\ (\Choice.get_metavar, \Tuple), \core.py\ (\Parameter, \Option), and \ ermui.py\ (\prompt) now use _convert_type.

\FuncParamType\ audit: no top-level import in \click/init.py, left unchanged to keep scope minimal.

Tests:

• New regression \ est_convert_type_privatized_with_deprecated_alias\ in \ ests/test_types/test_convert_type.py: watched FAIL before fix (\hasattr(click.types, '_convert_type')\ AssertionError), PASS after.
• Existing behavior tests switched to canonical _convert_type\ (required: pytest \ilterwarnings=error).
• \pytest tests\ (PYTHONPATH=src): 1976 passed, 108 skipped, 1 xfailed.
• 
  uff check\ +
  uff format --check: clean. \mypy\ (strict): no errors in touched files (7 pre-existing errors in untouched _termui_impl.py/\ esting.py, identical on baseline).

Note: run with \PYTEST_DISABLE_PLUGIN_AUTOLOAD=1\ + \PYTHONPATH=src\ because the sandbox interpreter has a broken global pytest plugin (langsmith/pydantic-core mismatch) and a non-editable click 8.5.0 install.

[davidism]

https://palletsprojects.com/contributing/llm-ai