Add comprehensive unit tests for private methods in pandas.core.nanops_numba

- Tests all 9 private methods prefixed with underscore
- 37 test cases organized in 8 test classes
- Comprehensive coverage of Numba-accelerated reduction operations
- Tests edge cases: NaN handling, empty arrays, masks, different dtypes
- Uses pytest fixtures and parameterization to avoid code duplication
- Tests NumbaList usage for parallel processing
- Uses pandas._testing for consistent assertion helpers
- All tests pass in pandas-dev environment

Functions tested:
- _get_initial_value: Finding first valid values in arrays
- _nb_reduce_single_arr: Single array reduction operations
- _nullify_below_mincount: Minimum count validation
- _reduce_empty_array: Empty array handling
- _chunk_arr_into_arr_list: Array chunking for parallel processing
- _nb_reduce_arr_list_in_parallel: Parallel reduction operations
- _reduce_chunked_results: Combining chunked results
- _cast_to_timelike: DateTime/timedelta type casting
- _nanvar_std_sem: Variance/standard deviation/standard error calculations

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: eoincondron