Handle non-string boundaries and add test cases for BaseMapper class

[Munanom]

Bugfix: handle non-string boundary inputs

• Add a check to ensure the boundary value is a string before attempting string operations
• Raise a ValueError with an appropriate error message when boundary is not a string
• This change prevents a TypeError from being raised when trying to process non-string boundary values

[Munanom]

Add test cases for BaseMapper class

• Add test_reading_geojson_and_loading_boundary to verify GeoJSON reading and boundary loading
• Add test_create_basemap_and_tiles to test basemap and tile creation
• Add test_load_boundary_with_bbox to test loading boundary with BBOX strings
• Add test_invalid_boundaries to test various invalid boundary inputs
• Add test_custom_tilemap_service to test customization of tilemap service
• Add test_imagery_sources to verify different imagery basemap sources
• Refactor existing test_create function into separate test cases
• Update imports and setup/teardown methods for testing

Draft testcases
Fixes: hotosm#203