fix(opencode): switch session models without restart

tests/unit/opencode-client.test.ts

@@ -169,6 +169,35 @@ describe('opencode client', () => {
     }
   });
 
+  it('switches the model used by subsequent turns through the OpenCode session API', async () => {
+    const fetchImpl = vi.fn(async () => new Response(null, { status: 204 }));
+    const client = createOpencodeClient({
+      baseUrl: 'http://127.0.0.1:4096',
+      directory: 'D:/work/app',
+      fetchImpl,
+    });
+    const controller = new AbortController();
+
+    await client.switchSessionModel('ses_1', {
+      providerID: 'niancode-user-models',
+      modelID: 'qwen3.7-plus',
+    }, { signal: controller.signal });
+
+    expect(fetchImpl).toHaveBeenCalledWith(
+      'http://127.0.0.1:4096/api/session/ses_1/model?directory=D%3A%2Fwork%2Fapp',
+      expect.objectContaining({
+        method: 'POST',
+        body: JSON.stringify({
+          model: {
+            providerID: 'niancode-user-models',
+            modelID: 'qwen3.7-plus',
+          },
+        }),
+        signal: controller.signal,
+      }),
+    );
+  });
+
   it('posts a text message to a session scoped to the selected folder', async () => {
     const fetchImpl = vi.fn(async () => {
       return new Response(JSON.stringify({ id: 'msg_1', role: 'user' }), {