fix(android): restore namespace fallback for libraries not applying the React plugin

#57038 narrowed configureNamespaceForLibraries from a rootProject-wide sweep to
only the project applying `com.facebook.react`. That was necessary because the old
code re-ran the sweep from every library, and AGP 9 errors when `finalizeDsl` is
registered after a project's DSL has been finalized.

The narrowing has a side effect: third-party Android libraries that do NOT apply
`com.facebook.react` and still rely on the manifest `package` attribute (e.g. older
react-native-linear-gradient) no longer receive a namespace, so they fail under
AGP 9 with "Namespace not specified".

Restore the broad coverage by sweeping all `com.android.library` subprojects once,
from the application project (which applies the plugin a single time, avoiding the
repeated-traversal finalizeDsl error from #57038). Guard configureNamespaceForLibraries
so it registers its finalizeDsl callback at most once per project, since libraries
that DO apply the React plugin are now reachable from both the per-library hook and
the app-level sweep.

Author: cortinico

packages/gradle-plugin/react-native-gradle-plugin/src/main/kotlin/com/facebook/react/ReactPlugin.kt

@@ -113,6 +113,20 @@ class ReactPlugin : Plugin<Project> {
       configureCodegen(project, extension, rootExtension, isLibrary = false)
       configureResources(project, extension)
       configureBuildTypesForApp(project)
+
+      // Apply the namespace fallback to every Android library in the build, including third-party
+      // libraries that don't apply the `com.facebook.react` plugin and still rely on the manifest
+      // `package` attribute (which AGP 9 no longer accepts as a namespace). The per-library hook
+      // below only covers libraries that apply our plugin, so we additionally sweep all library
+      // subprojects from the app. We do this once, from the application project, because re-running
+      // a rootProject-wide traversal from every library breaks on AGP 9, which errors when
+      // `finalizeDsl` is registered after a project's DSL has been finalized.
+      // See https://github.com/facebook/react-native/pull/57038.
+      project.rootProject.allprojects { subproject ->
+        subproject.pluginManager.withPlugin("com.android.library") {
+          configureNamespaceForLibraries(subproject)
+        }
+      }
     }
 
     // Library Only Configuration

packages/gradle-plugin/react-native-gradle-plugin/src/main/kotlin/com/facebook/react/utils/AgpConfiguratorUtils.kt

@@ -106,6 +106,15 @@ internal object AgpConfiguratorUtils {
   }
 
   fun configureNamespaceForLibraries(project: Project) {
+    // This helper can be reached both from a library's own application of the React plugin and
+    // from the app-level sweep in ReactPlugin (which also covers libraries that don't apply
+    // `com.facebook.react`). A project's `finalizeDsl` callback must be registered before AGP
+    // finalizes its DSL — registering it twice (or after finalization) is a hard error on AGP 9+ —
+    // so we guard to register the namespace fallback at most once per project.
+    if (project.extensions.extraProperties.has(NAMESPACE_CONFIGURED_PROPERTY)) {
+      return
+    }
+    project.extensions.extraProperties.set(NAMESPACE_CONFIGURED_PROPERTY, true)
     project.extensions.getByType(LibraryAndroidComponentsExtension::class.java).finalizeDsl { ext ->
       if (ext.namespace == null) {
         val manifestFile =
@@ -122,6 +131,8 @@ internal object AgpConfiguratorUtils {
   }
 }
 
+private const val NAMESPACE_CONFIGURED_PROPERTY = "com.facebook.react.internal.namespaceConfigured"
+
 const val DEFAULT_DEV_SERVER_PORT = "8081"
 
 fun getPackageNameFromManifest(manifest: File): String? {