增加模型选择功能，支持不同 Agent 模型

src/lib/chatStream.test.ts

@@ -51,6 +51,7 @@ describe("streamAgentChat", () => {
 
     await streamAgentChat({
       message: "hi",
+      model: "deepseek/deepseek-v4-pro",
       onEvent: (event) => events.push(event),
     });
 
@@ -60,6 +61,11 @@ describe("streamAgentChat", () => {
         method: "POST",
         projectHeaderMode: "include",
         skipAuthRedirect: true,
+        body: JSON.stringify({
+          message: "hi",
+          session_id: undefined,
+          model: "deepseek/deepseek-v4-pro",
+        }),
       }),
     );