This PR fixes function calling support for Deepseek V3 models by:

1. Adding Deepseek to the list of supported function calling models
2. Modifying the message conversion logic to handle Deepseek's format requirements
3. Updating the LLM class to pass model information to the converter

Deepseek's API expects message content to be a string, but we were sending a list of dictionaries with 'type' and 'text' keys. This fix converts the message format appropriately when using Deepseek models.

Fixes #5818

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:faeae80-nikolaik   --name openhands-app-faeae80   docker.all-hands.dev/all-hands-ai/openhands:faeae80