Fix Structured Output Logic, Add Optional json_object for supporting o1 Models, and Disable Streaming for Pydantic Models

[nikhil-pandey]

• New supports_json_output Property: Adds a property to models to specify if json_object mode is supported.
• Improved Fallback Logic for Structured Output: Fixes an issue where fallback logic for structured outputs would return None instead of a valid output.
• Disables Streaming for Pydantic Response Models: Updates the agent to avoid returning a stream when a response_model (using Pydantic) is specified, as streaming is incompatible with Pydantic models. The UI currently expects streaming, and adjustments will be required to support this change.

[ashpreetbedi]

thanks @nikhil-pandey, really great PR. Reviewing + testing over the next few days