Add unit tests for LLM providers and Chat Service Manager

[bhf]

Unit tests are needed to ensure the Chat service and LLM providers function correctly with mocked API responses, especially since src/lib/llm-providers/openai.ts currently has 0% coverage.

Scope

• Mock OpenAI API responses for the openai.ts provider.
• Test src/lib/chat-service-manager.ts for handling multiple providers and session state.

Acceptance Criteria

• Create src/lib/llm-providers/__tests__/openai.test.ts.
• Create src/lib/__tests__/chat-service-manager.test.ts.
• Verify streaming and non-streaming responses from providers.
• Test session persistence logic in the manager.