[Fizz] Should be considered complete inside onShellReady callback (facebook#33295)

We decremented `allPendingTasks` after invoking `onShellReady`. Which
means that in that scope it wasn't considered fully complete.

Since the pattern for flushing in Node.js is to start piping in
`onShellReady` and that's how you can get sync behavior, this led us to
think that we had more work left to do. For example we emitted the
`writeShellTimeInstruction` in this scenario before.

Author: sebmarkbage

packages/react-dom/src/__tests__/ReactDOMFizzServerNode-test.js

@@ -67,6 +67,21 @@ describe('ReactDOMFizzServerNode', () => {
     expect(output.result).toMatchInlineSnapshot(`"<div>hello world</div>"`);
   });
 
+  it('flush fully if piping in on onShellReady', async () => {
+    const {writable, output} = getTestWritable();
+    await act(() => {
+      const {pipe} = ReactDOMFizzServer.renderToPipeableStream(
+        <div>hello world</div>,
+        {
+          onShellReady() {
+            pipe(writable);
+          },
+        },
+      );
+    });
+    expect(output.result).toMatchInlineSnapshot(`"<div>hello world</div>"`);
+  });
+
   it('should emit DOCTYPE at the root of the document', async () => {
     const {writable, output} = getTestWritable();
     await act(() => {

packages/react-server/src/ReactFizzServer.js

@@ -4335,6 +4335,7 @@ function finishedTask(
   boundary: Root | SuspenseBoundary,
   segment: null | Segment,
 ) {
+  request.allPendingTasks--;
   if (boundary === null) {
     if (segment !== null && segment.parentFlushed) {
       if (request.completedRootSegment !== null) {
@@ -4417,7 +4418,6 @@ function finishedTask(
     }
   }
 
-  request.allPendingTasks--;
   if (request.allPendingTasks === 0) {
     completeAll(request);
   }