Create comprehensive configuration loading test suite

[bdamokos]

Create comprehensive configuration loading test suite

Issue Description

We need a comprehensive test suite that examines all possible combinations of provider configurations across different configuration sources. This will help prevent issues like #17 (BKK config loading order) and ensure consistent behavior across all providers.

Test Cases to Cover

The test suite should verify configuration loading for:

1. All implemented providers (STIB, De Lijn, BKK)
2. All configuration sources:
   • Provider defaults (app/transit_providers/*/config.py)
   • Default configuration (default.py)
   • Local configuration (local.py)
3. All combinations of:
   • Full configuration in each source
   • Partial configuration in each source
   • Missing configuration in each source
   • Overlapping configurations

Implementation Details

1. Create a test framework that can:
   • Modify configuration files programmatically
   • Start/stop the server
   • Make API requests
   • Verify the returned configuration
2. Test scenarios should include:
   • Basic configuration loading
   • Configuration merging
   • Configuration overriding
   • Default fallbacks
   • Invalid configurations

Success Criteria

• All providers follow the same configuration precedence rules
• Configuration changes in any source are correctly reflected in the API
• Invalid configurations are handled gracefully
• Tests are automated and can be run as part of CI/CD

Related Issues

• 🐛 BKK config loading order not following expected precedence #17 BKK config loading order issue