Add "reasoning" attribute to AIMessage

[afirstenberg]

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Proposal is to add an attribute to AIMessage:

  reasoning?: MessageContent;

See discussion on Slack (https://langchaincommunity.slack.com/archives/C07EHF3HC87/p1737150066310499) and discussion in #7434 and #7564

cc: @jacoblee93

[jacoblee93]

We are going to put reasoning_content in additional_kwargs for now but will upgrade the schema in the future:

#7604

[afirstenberg]

@jacoblee93 - What is the format of reasoning_content? Whatever the model returns or should we be defining a specific type?

[jacoblee93]

For now whatever the model defines (Deepseek makes it a string)

Would imagine in the future it'll be content blocks or string but would prefer to wait for the dust to settle a bit