refactor: move agents.chat.ReAct to the chat package

What does this merge request do and why?

Small MR to clean up our agents package after moving all Yaml definitions to the dedicated definitions folder.

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.