feat: answer routes re-structure for better maintainability and reuse

[siiddhantt]

• Added BaseAnswerResource class for shared logic across answer endpoints.
• Created StreamResource for handling streaming responses based on user queries.
• Introduced ConversationService for managing conversation data in MongoDB.
• Developed StreamProcessor to initialize agents and retrievers, and handle conversation history.
• Updated user attachment storage to support API key authentication.
• Refactored utility functions for better model retrieval and chat history management.
• Removed deprecated extensions and adjusted imports for cleaner structure.
• Modified frontend handlers to remove unnecessary history parameter passing.

[vercel]

@siiddhantt is attempting to deploy a commit to the Arc53 Team on Vercel.

A member of the Team first needs to authorize it.

[dartpain]

All good, thank you!