feat(agent): route chat through selected model

src/modules/agent/messages.ts

@@ -1,4 +1,5 @@
 import { z } from "zod";
+import { isValidLlmModelId } from "@/lib/llm/models";
 
 export const clientChatAttachmentSchema = z.object({
   type: z.literal("image"),
@@ -11,8 +12,14 @@ export const clientChatMessageSchema = z.object({
   attachments: z.array(clientChatAttachmentSchema).max(3).optional(),
 });
 
+export const clientChatModelSchema = z
+  .string()
+  .trim()
+  .refine((value) => isValidLlmModelId(value), "Invalid assistant model");
+
 export const clientChatInputSchema = z.object({
   stream: z.boolean().optional(),
+  model: clientChatModelSchema.optional(),
   messages: z.array(clientChatMessageSchema).min(1).max(40),
 });