fix[devtools/updateFiberRecursively]: mount suspense fallback set in timed out case (#27147)

Fixes #26793.

I have received a constantly reproducible example of the error, that is
mentioned in the issue above.
When starting `Reload and Profile` in DevTools, React reports an unmount
of a functional component inside Suspense's fallback via
[`onCommitFiberUnmount`](https://github.com/facebook/react/blob/3ff846d106de9273f59d1e4457793a5fcf625aef/packages/react-devtools-shared/src/hook.js#L408-L413)
in
[`commitDeletionEffectsOnFiber`](https://github.com/facebook/react/blob/main/packages/react-reconciler/src/ReactFiberCommitWork.js#L2025),
but this fiber was never registered as mounted in DevTools.

While debugging, I've noticed that in timed-out case for Suspense trees
we only check if both previous fallback child set and next fiber
fallback child set are non-null, but in these recursive calls there is
also a case when previous fallback child set is null and next set is
non-null, so we were skipping the branch.

<img width="1746" alt="Screenshot 2023-07-25 at 15 26 07"
src="https://github.com/facebook/react/assets/28902667/da21a682-9973-43ec-9653-254ba98a0a3f">

After these changes, the issue is no longer reproducible, but I am not
sure if this is the right solution, since I don't know if this case is
correct from reconciler perspective.

Author: hoxyq

packages/react-devtools-shared/src/backend/renderer.js

@@ -2345,6 +2345,18 @@ export function attach(
       const prevFallbackChildSet = prevFiberChild
         ? prevFiberChild.sibling
         : null;
+
+      if (prevFallbackChildSet == null && nextFallbackChildSet != null) {
+        mountFiberRecursively(
+          nextFallbackChildSet,
+          shouldIncludeInTree ? nextFiber : parentFiber,
+          true,
+          traceNearestHostComponentUpdate,
+        );
+
+        shouldResetChildren = true;
+      }
+
       if (
         nextFallbackChildSet != null &&
         prevFallbackChildSet != null &&

packages/react-devtools-shared/src/hook.js

@@ -320,7 +320,7 @@ export function installHook(target: any): DevToolsHook | null {
 
   let uidCounter = 0;
 
-  function inject(renderer: ReactRenderer) {
+  function inject(renderer: ReactRenderer): number {
     const id = ++uidCounter;
     renderers.set(id, renderer);