Feature/OpenAI reasoning field

[AlanPonnachan]

Support sending thinking parts in custom request fields for OpenAI-compatible models

Closes: #3446

This PR implements the ability to send reasoning/thinking content in a specific field of the request message (e.g., reasoning_content) for OpenAI-compatible providers that support it, such as Moonshot AI.

Currently, OpenAIChatModel embeds thinking parts into the content string using tags (e.g. <think>...</think>). However, providers like Moonshot recommend sending this data in a separate field to allow the model to process context more effectively.

Changes:

• pydantic_ai_slim/pydantic_ai/profiles/openai.py: Added openai_chat_reasoning_field to OpenAIModelProfile.
• pydantic_ai_slim/pydantic_ai/models/openai.py: Updated OpenAIChatModel to check this profile setting. If set, ThinkingPart content is collected and injected into the specified field of the message payload (e.g., messages[i]['reasoning_content']) instead of being appended to content.
• pydantic_ai_slim/pydantic_ai/profiles/moonshotai.py: Updated the Moonshot AI profile to use reasoning_content.
• tests/models/test_openai.py: Added test_openai_chat_reasoning_field to verify that the extra field is correctly present in the API request arguments.

[ZeroAurora]

I was working on the same logic in my branch XD
I still have some test work left there though

[DouweM]

@AlanPonnachan The other PR by @ZeroAurora is further along so I'll go with that one; still appreciate the effort!