Add tests for error handling and invalid configurations in `config.py`

[lamurian]

User Story

As a developer, I want to add tests for src/python/config.py that simulate invalid configurations and missing environment variables so that all error handling and edge case paths are properly validated.

Description

config.py currently has 81% test coverage. The missing 19% corresponds to error conditions and validation failures. Existing tests only cover valid configurations, leaving fallback and error-handling logic untested. New tests should simulate missing environment variables, invalid values, and incomplete configuration objects to ensure robustness.

Proposed Flow

• Add tests for missing python-dotenv fallback (Lines 51–52)
• Test invalid environment variable handling (Lines 76, 87)
• Test missing configuration keys (Lines 274, 277, 280)
• Add tests for parameter validation failures (Lines 295–332)
• Ensure all tests trigger the appropriate exceptions or error messages

Reference

• Current coverage report for src/python/config.py
• Environment variable handling logic and parameter validation rules

Notes:

• Tests should be isolated and not rely on actual environment state; use mocking where needed.
• Validate both exception raising and fallback mechanisms.
• Ensure coverage report reflects execution of all missing error paths.