Route Harmony reasoning separately from final output (#21378)

[Gasoonjia]

Summary:

Teach the shared OpenAI chat adapter to expose optional reasoning_content.

Reviewed By: metascroy

Differential Revision: D112991739

[pytorch-bot]

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[meta-codesync]

@Gasoonjia has exported this pull request. If you are a Meta employee, you can view the originating Diff in D112991739.

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