Add input validation for dictionary -form region parameters in _parse_single_region and add mini-test to it

[Batmend-bit]

Summary

This PR adds input validation for the 'Mapping' code path in '_parse_single_region'. In the previous code, there was no problem only if the inputs were perfectly safe. However, there was no input validation; malformed dictionary inputs can be silently accepted and would cause issues later. This change makes the function fail fast with a clear, actionable error message at the point of entry.

Problem

The following malformed inputs can be silently accepted:

1. Missing contig: {"start":100, "end":200}
2. Invalid contig name: {"contig": "Invalid_name", "start':100, "end":200}
3. Negative coordinate or end > start: {"contig": "2L", "start':-100, "end":200} or {"contig": "2L", "start':100, "end": 20}
4. Ending coordinate is greater than the length of the genome sequence.
   Therefore, I thought that input validation for the 'Mapping' is useful.

Solution

Added the following validation in the 'isinstance(region, Mapping)' branch:

1. 'contig' is required - raises ValueError if missing
2. 'contig' must be valid - checked against '_valid_contigs(resource)' as same as how string regions are validated in '_handle_region_coords'
3. Coordinates must be positive integers - checked as same as in '_handle_region_coords'
4. 'start' must not exceed 'end' - consistent with the existing check in
   _handle_region_coords
5. 'end' must be less than the length of the sequence
   Additionally added a docstring to clarify expected input structure and validation behavior.

Effect

1. Fully backward compatible for valid inputs.
2. No existing tests are broken.
3. Just added validation for inputs that were already broken to prevent silent acceptance of malformed inputs.

Testing

New tests were added in 'tests/test_util.py' with pytest by covering:

• Valid whole-contig dict region
• Valid interval dict region (only end, only start coordinates are accepted)
• 8 invalid dict cases each raising ValueError
• Invalid string region cases
• Invalid type cases raising TypeError

All tests passed when I ran it with poetry: poetry run pytest tests/test_util.py -v