api: structured outputs - chat endpoint (#7900)

Adds structured outputs to chat endpoint
---------

Co-authored-by: Michael Yang <mxyng@pm.me>
Co-authored-by: Hieu Nguyen <hieunguyen1053@outlook.com>

server/routes.go

@@ -278,7 +278,7 @@ func (s *Server) GenerateHandler(c *gin.Context) {
 		if err := r.Completion(c.Request.Context(), llm.CompletionRequest{
 			Prompt:  prompt,
 			Images:  images,
-			Format:  req.Format,
+			Format:  json.RawMessage(req.Format),
 			Options: opts,
 		}, func(cr llm.CompletionResponse) {
 			res := api.GenerateResponse{