fix: validate research structured input before request

[Shedna]

Description

• add the missing client-side validation that requires structured_output_schema when calling research(..., output_type=structured)
• keep sync and async research behavior aligned with the platform DTO contract and the existing search validation path
• add unit tests for the sync and async research validation cases

Checklist

• I have run make lint on my changes.
• I have run make typecheck test on my changes.
• I have updated README.md if my changes affected it.