Add unit tests for config module

[MarkEdmondson1234]

Adds unit tests for the load_config_key function in the sunholo.utils.config module to enhance the test coverage of the sunholo-py library.

• New Tests: Implements tests for the load_config_key function, covering scenarios for existing keys, non-existing keys, and invalid configurations.
• Mocking: Utilizes mocking to simulate the load_all_configs function's return values for different test cases.

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:rocket:	This description was created by Ellipsis for commit ecc05402179b31252de54239b398bc6e0a5d9337

Summary:

This PR introduces unit tests for the load_config_key function in the sunholo.utils.config module, enhancing test coverage and error handling by simulating various scenarios.

Key points:

• Adds unit tests for the load_config_key function in the sunholo.utils.config module
• Covers scenarios for existing keys, non-existing keys, and invalid configurations
• Utilizes mocking to simulate load_all_configs function's return values
• Changes error handling to raise KeyError for invalid configurations

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