#739 fix: safely disable mem0 for non-OpenAI models

• Use getattr to safely check for model_name attribute, resolving Pylance type warnings.
• Automatically disable vector memory for non-OpenAI LLMs to prevent API key errors.
• Add assertion to ensure main_llm is initialized before agent creation.

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Summary by cubic

Safely disables mem0 vector memory for non-OpenAI models and ensures the LLM is initialized before creating the agent. Prevents OpenAI API key errors and resolves type-check warnings.

• Bug Fixes
  • Use getattr to read model_name safely, avoiding Pylance warnings.
  • Detect non-OpenAI models using llm_provider_name when available; disable vector memory and log a clear warning.
  • Assert main_llm is set before agent creation and pass the enable_memory flag to the agent.