[BUG] Fix json_schema method in the with_structured_output function.

[paxiaatucsdedu]

Problem

Issue link: #40

1. with_structured_output() use method="function_calling" as default method. When calling Gemini 2.5 models, no function calling is returned so with_structured_output() does not work with Gemini 2.5 models.
2. If using the json_schema method in with_structured_output() , with_structured_output() was failing with a 400 error ("Please pass in correct format of request"). The root cause is that the code was passing a plain Python dict for response_format instead of proper OCI SDK objects that the GenericChatRequest API expects.

Solution

1. Suggest in README that user should use json_schema method when the default method does not work.
2. Updated the json_schema method to use proper OCI SDK objects:

• ResponseJsonSchema - wraps the JSON schema with name, description, and strict mode
• JsonSchemaResponseFormat - the proper response format type expected by the API

Testing

Now Gemini-2.5 model returns properly formatted object instead of None when using json_schema method in with_structured_output() function.

[YouNeedCryDear]

Can you double check if the langchain openai is still using function_calling as default? Changing default might introduce unexpected behavior for customer.

[paxiaatucsdedu]

1. langchain openai is still using function_calling as default. See reference:
   https://github.com/langchain-ai/langchain/blob/master/libs/partners/openai/langchain_openai/chat_models/base.py

2. The note in our repo said:
   If "json_schema" then it allows the user to pass a json schema (or pydantic) to the model for structured output. This is the default method.

That is why I change the default method to json_schema. I thought using function_calling as default was a typo.

[YouNeedCryDear]

That's probably a mistake in the docstring. Let's not update the default and move that "This is the default method." to the correct place.

[paxiaatucsdedu]

Of course. I have changed the default back to function calling. Also commented in README that please use json_schema if the default does not work.