fix: correct get_metadata return annotations

[Gagandeep-2003]

Summary

• Correct the return annotations of DataFrameModel.get_metadata() and DataFrameSchema.get_metadata() from dict | None to dict[Any, Any].
• Add static typing regression coverage using Pandera’s existing no-plugin mypy test setup.

Both methods always return dictionaries, so callers should not need to handle an impossible None value. This change only corrects the public type annotations and does not alter runtime behavior.

Closes #1750

Tests

• • .venv/bin/python -m mypy tests/mypy/pandas_modules/pandera_metadata.py --config-file tests/mypy/config/no_plugin.ini

• .venv/bin/python -m pytest tests/mypy/test_pandas_static_type_checking.py -q
• .venv/bin/python -m pytest tests/pandas/test_schemas.py::test_get_schema_metadata tests/pandas/test_model.py::test_pandas_fields_metadata -q
• .venv/bin/python -m pytest tests/pandas/test_model.py tests/pandas/test_schemas.py -q
• prek run --files pandera/api/dataframe/container.py pandera/api/dataframe/model.py tests/mypy/pandas_modules/pandera_metadata.py tests/mypy/test_pandas_static_type_checking.py