refactor: set model parameters in the agent definition file

What does this merge request do and why?

This MR implements the logic to set basic model parameters in the YAML definition file of the agent.

Blocked by !1028 (merged)

How to set up and validate locally

1. Check out to this merge request's branch.
2. Ensure a local Docker image built successfully.

   docker build --platform linux/amd64 -t ai-gateway:dev .

3. Run a local service on Docker.

   docker run --platform linux/amd64 --rm \
     -p 5052:5052 \
     -e AIGW_AUTH__BYPASS_EXTERNAL=true \
     -v $PWD:/app -it ai-gateway:dev

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.