Add AIMessage and AIMessageChunk attribute Reasoning Content

[lanhui100]

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Feature request

Reasoning models are on the rise, and the generation process will include reasoning content. It is hoped that the AIMessage and AIMessageChunk instances can have a reasoning_content attribute added.

Motivation

Applications can showcase the model's thought process.

Proposal (If applicable)

No response

[yaoye7758521]

good idea

[lanhui100]

I mean specific in stream mode.

[bzr1]

also in BaseChatOpenAI(),ChatOpenAI(), and chain please