feat: v1/messages support, part 1 (types): add reasoning field to ChatMessage

[RobGeada]

Description

Add an optional reasoning field to the ChatMessage dataclass so reasoning content survives round-tripping through to_dict/from_dict, and extract it from OpenAI-compatible responses via the 'reasoning' key in addition to 'reasoning_content'. This is in preparation for the inline reasoning blocks inside of Anthropic v1/messages. Incidentally, this should also add vLLM reasoning support, which arrives in the reasoning field of a message.

Related Issue(s)

#2226

Verification

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Checklist

• I've read the CONTRIBUTING guidelines.
• This PR links to a triaged issue assigned to me.
• My PR title follows the project commit convention.
• I've updated the documentation if applicable.
• I've added tests if applicable.
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[github-actions]

PR merge guidance

@RobGeada thanks for the PR. GitHub is currently blocking merge for one or more repository requirements:

• 1 commit does not have a verified signature (26e6b0e). Please sign the commits and force-push the updated branch.

Relevant guide:

• Signed commits: https://github.com/NVIDIA-NeMo/Guardrails/blob/develop/CONTRIBUTING.md#commit-signing
• Contribution guide: https://github.com/NVIDIA-NeMo/Guardrails/blob/develop/CONTRIBUTING.md