add response headers for model tracking and observability

[smtdfc]

Description

This PR adds custom response headers to the handleComboChat function to improve observability for downstream clients.

Key Changes

• x-9router-combo: Set to true for all combo requests.
• x-9router-model: Returns the name of the model that handled the request.

Example Response Headers

HTTP/1.1 200 OK
Content-Type: application/json
x-9router-combo: true
x-9router-model: gpt-4o-mini