fix(backend): validate configured model provider pair

backend/test/routes-settings.test.ts

@@ -64,6 +64,33 @@ test("PUT /settings rejects an unknown model when a model list is available", as
   }
 });
 
+test("PUT /settings validates provider and model as one composite identifier", async () => {
+  const { app, dir } = appWithTmpSettings({}, {
+    listModels: async () => [
+      { provider: "provider-a", id: "shared-id", name: "A", reasoning: false },
+    ],
+  });
+  try {
+    const wrongProvider = await app.inject({
+      method: "PUT", url: "/settings",
+      payload: {
+        workspace: "psd", provider: "provider-b", model: "shared-id", thinking: "low",
+      },
+    });
+    expect(wrongProvider.statusCode).toBe(400);
+
+    const exactPair = await app.inject({
+      method: "PUT", url: "/settings",
+      payload: {
+        workspace: "psd", provider: "provider-a", model: "shared-id", thinking: "low",
+      },
+    });
+    expect(exactPair.statusCode).toBe(200);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
 test("PUT /settings allows any model when model list is empty (Pi unavailable)", async () => {
   const { app, dir } = appWithTmpSettings({}, { listModels: async () => [] });
   try {