feat(agent): route chat through selected model

tests/unit/agent-messages.test.ts

@@ -25,4 +25,22 @@ describe("clientChatInputSchema", () => {
 
     assert.equal(parsed.success, false);
   });
+
+  it("accepts an optional model ID", () => {
+    const parsed = clientChatInputSchema.safeParse({
+      model: "qwen2.5-coder",
+      messages: [{ role: "user", content: "hello" }],
+    });
+
+    assert.equal(parsed.success, true);
+  });
+
+  it("rejects invalid model IDs", () => {
+    const parsed = clientChatInputSchema.safeParse({
+      model: "bad model",
+      messages: [{ role: "user", content: "hello" }],
+    });
+
+    assert.equal(parsed.success, false);
+  });
 });