[CQ] API: migrate FlutterSdkManager off projectOpened (#8123)

Move `FlutterSdkManager` (unstable) post-project open initialization to
a (supported) `ProjectActivity`.

See: #8100,
#6953

---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

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  <summary>Contribution guidelines:</summary><br>

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Author: pq

flutter-idea/src/io/flutter/sdk/FlutterProjectActivity.kt

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2025 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+package io.flutter.sdk
+
+import com.intellij.openapi.project.Project
+import com.intellij.openapi.startup.ProjectActivity
+
+class FlutterProjectActivity : ProjectActivity {
+  override suspend fun execute(project: Project) {
+    FlutterSdkManager.getInstance(project).checkForFlutterSdkChange()
+  }
+}

flutter-idea/src/io/flutter/sdk/FlutterSdkManager.java

@@ -52,20 +52,6 @@ private FlutterSdkManager(@NotNull Project project) {
     });
 
     ProjectManager.getInstance().addProjectManagerListener(myProject, new ProjectManagerListener() {
-      //See https://plugins.jetbrains.com/docs/intellij/plugin-components.html#comintellijpoststartupactivity
-      // for notice and documentation on the deprecation intentions of
-      // Components from JetBrains.
-      //
-      // Migration forward has different directions before and after
-      // 2023.1, if we can, it would be prudent to wait until we are
-      // only supporting this major platform as a minimum version.
-      //
-      // https://github.com/flutter/flutter-intellij/issues/6953
-      @Override
-      public void projectOpened(@NotNull Project project) {
-        checkForFlutterSdkChange();
-      }
-
       @Override
       public void projectClosed(@NotNull Project project) {
         checkForFlutterSdkChange();

resources/META-INF/plugin.xml

@@ -361,6 +361,7 @@
                     serviceImplementation="io.flutter.bazel.WorkspaceCache"/>
     <projectService serviceImplementation="io.flutter.pub.PubRootCache"/>
 
+    <backgroundPostStartupActivity implementation="io.flutter.sdk.FlutterProjectActivity"/>
     <backgroundPostStartupActivity id="FontPreviewStartupActivity"
                                    implementation="io.flutter.font.FontPreviewStartupActivity"
                                    order="last"/>

resources/META-INF/plugin_template.xml

@@ -269,6 +269,7 @@
                     serviceImplementation="io.flutter.bazel.WorkspaceCache"/>
     <projectService serviceImplementation="io.flutter.pub.PubRootCache"/>
 
+    <backgroundPostStartupActivity implementation="io.flutter.sdk.FlutterProjectActivity"/>
     <backgroundPostStartupActivity id="FontPreviewStartupActivity"
                                    implementation="io.flutter.font.FontPreviewStartupActivity"
                                    order="last"/>