Exclude legacy Hermes targets from single host assertion (#54784)

Summary:
Pull Request resolved: #54784

This is a follow-up to D86201689 following user feedback on `0.83.0-rc.2`: react-native-community/discussions-and-proposals#954 (reply in thread).

The problem was that we were incorrectly attributing any and all `IInspector::addPage()` calls to represent a new React Native Host being registered. However, this API also remains common to the legacy Hermes `ConnectionDemux` runtime target setup (which does not represent a host target).

We now fork this code path internally based on `InspectorTargetCapabilities.prefersFuseboxFrontend`, in order to track `registeredHostsCount` accurately (i.e. **only** for HostTarget page registrations).

Changelog:
[General][Fixed] - React Native DevTools: Fix a bug where we would incorrectly flag apps using additonal Hermes runtimes (e.g. Reanimated) as being multi-host

Reviewed By: hoxyq

Differential Revision: D88386623

fbshipit-source-id: 49a82faa6c857de41e8e9a757f5fdd850f09c3dd

Author: huntie

packages/react-native/ReactCommon/jsinspector-modern/InspectorInterfaces.cpp

@@ -143,9 +143,13 @@ int InspectorImpl::addPage(
       pageId,
       Page{pageId, description, vm, std::move(connectFunc), capabilities});
 
-  for (const auto& listenerWeak : listeners_) {
-    if (auto listener = listenerWeak.lock()) {
-      listener->unstable_onHostTargetAdded();
+  // Strong assumption: If prefersFuseboxFrontend is set, the page added is a
+  // HostTarget and not a legacy Hermes runtime target.
+  if (capabilities.prefersFuseboxFrontend) {
+    for (const auto& listenerWeak : listeners_) {
+      if (auto listener = listenerWeak.lock()) {
+        listener->unstable_onHostTargetAdded();
+      }
     }
   }