test(feedback): mock Cohere client in test fixtures and allow optional client injection

[ravimahatonp]

Summary

This PR enables tests to execute without requiring a real COHERE_API_KEY environment variable. Previously, running tests in an environment without COHERE_API_KEY resulted in a pydantic ValidationError upon importing settings. Furthermore, FeedbackEngine instantiated a live CohereClient unconditionally in __init__.

Changes Made

• src/config.py: Provided a safe default empty string for cohere_api_key so that importing configuration and services does not fail validation during unit testing.
• src/services/feedback_engine.py: Supported client dependency injection via __init__(self, client: Any = None) so that mock clients can be passed cleanly.
• tests/test_feedback_engine.py: Injected MagicMock() client into the FeedbackEngine fixtures for TestFeedbackScoreComputation and TestQuickFeedback, decoupling unit tests from external Cohere client instantiation.

Closes #10