Chat LLM call refactor

[zmccormick7]

Chat LLM call updates

• upgrades Gemini SDK to newer google-genai version
• moves get_response code to chat folder, since that's the only place it's used
• changes model name specification to {provider}/{model_name} (old method remains for backwards compatibility)
• removes unused non-Instructor handler functions
• adds special handling needed for OpenAI o-series models

Unrelated

• removes Milvus integration test because the MilvusDB class requires special configuration to run via GitHub Actions