fix(dev-middleware): respond with status code 200 when launching RNDT (#46814)

Summary:
This fixes an issue where `POST /open-debugger?appId&device&target` does not return a proper status code, meaning that the request will never be answered and clients might hang until the request timeout is hit.

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[GENERAL] [FIXED] - Respond with status code `200` when successfully launching RNDT

Pull Request resolved: https://github.com/facebook/react-native/pull/46814

Test Plan:
- `curl -v -X POST "<deviceUrl>"`
- This should show a proper response for the request.

before | after
 --- | ---
![image](https://github.com/user-attachments/assets/5b820acd-1168-4642-90ec-f2eeec0afc16) | ![image](https://github.com/user-attachments/assets/82bb2a6c-3c7b-483f-a4a1-ad00e5ca0178)

Reviewed By: NickGerleman

Differential Revision: D63837025

Pulled By: huntie

fbshipit-source-id: ac72fc793e015f0eec498f4a35b4fb9e301c5b32

packages/dev-middleware/src/__tests__/FetchUtils.js

@@ -21,7 +21,7 @@ declare var globalThis: $FlowFixMe;
  */
 export async function fetchLocal(
   url: string,
-  options?: Parameters<typeof fetch>[1] & {dispatcher?: mixed},
+  options?: Partial<Parameters<typeof fetch>[1] & {dispatcher?: mixed}>,
 ): ReturnType<typeof fetch> {
   return await fetch(url, {
     ...options,

packages/dev-middleware/src/__tests__/InspectorProxyHttpApi-test.js

@@ -18,6 +18,7 @@ import {fetchJson, fetchLocal} from './FetchUtils';
 import {createDeviceMock} from './InspectorDeviceUtils';
 import {withAbortSignalForEachTest} from './ResourceUtils';
 import {withServerForEachTest} from './ServerUtils';
+import DefaultBrowserLauncher from '../utils/DefaultBrowserLauncher';
 
 // Must be greater than or equal to PAGES_POLLING_INTERVAL in `InspectorProxy.js`.
 const PAGES_POLLING_DELAY = 1000;
@@ -368,4 +369,60 @@ describe('inspector proxy HTTP API', () => {
       }
     });
   });
+
+  describe('/open-debugger endpoint', () => {
+    it('opens requested device using appId, device, and target', async () => {
+      // Connect a device to use when opening the debugger
+      const device = await createDeviceMock(
+        `${serverRef.serverBaseWsUrl}/inspector/device?device=device1&name=foo&app=bar`,
+        autoCleanup.signal,
+      );
+      device.getPages.mockImplementation(() => [
+        {
+          app: 'bar-app',
+          id: 'page1',
+          title: 'bar-title',
+          vm: 'bar-vm',
+          capabilities: {
+            // Ensure the device target can be found when launching the debugger
+            nativePageReloads: true,
+          },
+        },
+      ]);
+      jest.advanceTimersByTime(PAGES_POLLING_DELAY);
+
+      // Hook into `DefaultBrowserLauncher.launchDebuggerAppWindow` to ensure debugger was launched
+      const launchDebuggerSpy = jest
+        .spyOn(DefaultBrowserLauncher, 'launchDebuggerAppWindow')
+        .mockResolvedValueOnce();
+
+      try {
+        // Fetch the target information for the device
+        const pageListResponse = await fetchJson<JsonPagesListResponse>(
+          `${serverRef.serverBaseUrl}/json`,
+        );
+        // Select the first target from the page list response
+        expect(pageListResponse.length).toBeGreaterThanOrEqual(1);
+        const firstPage = pageListResponse[0];
+
+        // Build the URL for the debugger
+        const openUrl = new URL('/open-debugger', serverRef.serverBaseUrl);
+        openUrl.searchParams.set('appId', firstPage.description);
+        openUrl.searchParams.set(
+          'device',
+          firstPage.reactNative.logicalDeviceId,
+        );
+        openUrl.searchParams.set('target', firstPage.id);
+        // Request to open the debugger for the first device
+        const response = await fetchLocal(openUrl.toString(), {method: 'POST'});
+
+        // Ensure the request was handled properly
+        expect(response.status).toBe(200);
+        // Ensure the debugger was launched
+        expect(launchDebuggerSpy).toHaveBeenCalledWith(expect.any(String));
+      } finally {
+        device.close();
+      }
+    });
+  });
 });

packages/dev-middleware/src/middleware/openDebuggerMiddleware.js

@@ -132,6 +132,7 @@ export default function openDebuggerMiddleware({
                 {launchId, useFuseboxEntryPoint},
               ),
             );
+            res.writeHead(200);
             res.end();
             break;
           case 'redirect':