Add StartThreadRequest Pydantic schema for thread/start endpoint

[avirajsingh7]

Summary

Target issue #216

This PR introduces a new Pydantic schema, StartThreadRequest, used to validate and structure incoming request data for the /thread/start endpoint.

Changes:
Added StartThreadRequest schema with the following fields:

• question (str): the user's query
• assistant_id (str): the OpenAI assistant ID
• remove_citation (bool): flag to control citation behavior (default False)

Checklist

Before submitting a pull request, please ensure that you mark these task.

• Ran fastapi run --reload app/main.py or docker compose up in the repository root and test.
• If you've fixed a bug or added code that is tested and has test cases.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/app/api/routes/threads.py	87.50%	1 Missing ⚠️

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