POST endpoints

[gekinci]

This PR updates the chatbot API. The clients now use explicit POST request bodies for all endpoints that expect query parameters. This eliminates the security risk of exposing sensitive information in the query parameters.

Changes:

• Converted chatbot endpoints from GET to POST with Pydantic request body models: getthread, getuserthreads, streamresponse, stop, editthread, deletethread, searchthreads, setthreadtopic, and userfeedback.
• Added custom OpenAPI security schemes for bearer auth and x-freva-rest-url, applying them to chatbot API operations so FastAPI docs expose the expected authorization inputs.
• Updated README endpoint documentation to reflect POST methods and JSON body parameters.
• Updated integration tests.

The list of endpoints and their method after the update:
POST: deletethread, editthread, getthread, getuserthreads, searchthreads, setthreadtopic, stop, streamresponse, userfeedback
GET: availablechatbots, ping, help

[gekinci]

@felio92 @BiancaWentzel can you take a look at this? Thanks