feat(apple): let the host app supply the FlutterDartProject for background engines

BackgroundWorker created its headless FlutterEngine with project: nil, so every
task built a fresh FlutterDartProject whose settings can differ from the main
engine's. Add WorkmanagerPlugin.setDartProject(_:) so the host app can share
one project with the background engines. Behavior is unchanged when it is not
called.

Author: zkrige

build.log

@@ -0,0 +1,14 @@
+Warning: Building for device with codesigning disabled. You will have to manually codesign before deploying to device.
+Building dev.fluttercommunity.workmanagerExample for device (ios)...
+All plugins found for ios are Swift Packages, but your project still has CocoaPods integration. Your project uses a non-standard Podfile and will need to be migrated to Swift Package Manager manually. Some steps you may need to complete include:
+  * In the ios/ directory run "pod deintegrate"
+  * Transition any Pod dependencies to Swift Package equivalents. See https://developer.apple.com/documentation/xcode/adding-package-dependencies-to-your-app
+  * Transition any custom logic
+  * Remove the include to "Pods/Target Support Files/Pods-Runner/Pods-Runner.debug.xcconfig" in your ios/Flutter/Debug.xcconfig
+  * Remove the include to "Pods/Target Support Files/Pods-Runner/Pods-Runner.release.xcconfig" in your ios/Flutter/Release.xcconfig
+
+Removing CocoaPods integration will improve the project's build time.
+Running pod install...                                             520ms
+Running Xcode build...                                          
+Xcode build done.                                           15,1s
+✓ Built build/ios/iphoneos/Runner.app

docs/quickstart.mdx

@@ -85,6 +85,19 @@ import workmanager_apple
 }
 ```
 
+Background engines create their own `FlutterDartProject` by default. If your
+app needs them to use the same project as the main engine (for example when a
+code push tool resolves the newest downloaded patch each time a project is
+created), create one project at launch, pass it to your engine, and call
+`WorkmanagerPlugin.setDartProject(_:)` with it before any task runs:
+
+```swift
+let project = FlutterDartProject()
+WorkmanagerPlugin.setDartProject(project)
+```
+
+When the call is omitted, behavior is unchanged.
+
 <Info>
 **UIScene lifecycle:** Apple requires UIKit apps to adopt the UIScene lifecycle
 from the release following iOS 26. After migration, call

workmanager_apple/ios/workmanager_apple/Sources/workmanager_apple/BackgroundWorker.swift

@@ -158,7 +158,7 @@ class BackgroundWorker {
 
         var flutterEngine: FlutterEngine? = FlutterEngine(
             name: backgroundMode.flutterThreadlabelPrefix,
-            project: nil,
+            project: WorkmanagerPlugin.dartProject,
             allowHeadlessExecution: true
         )
 

workmanager_apple/ios/workmanager_apple/Sources/workmanager_apple/WorkmanagerPlugin.swift

@@ -45,6 +45,22 @@ public class WorkmanagerPlugin: WorkmanagerPluginBase, FlutterPlugin, Workmanage
     /// (fluttercommunity/flutter_workmanager#653).
     static var mainBinaryMessenger: FlutterBinaryMessenger?
 
+    static var dartProject: FlutterDartProject?
+
+    /// Sets the `FlutterDartProject` used by the background engines.
+    ///
+    /// Without this call each background engine creates its own project. Apps
+    /// whose project settings change while the app runs (for example code push
+    /// tools that resolve the newest downloaded patch when a project is
+    /// created) should create one project at launch and share it with the main
+    /// engine and with this plugin.
+    ///
+    /// - Parameter project: The project the host app created at launch
+    @objc
+    public static func setDartProject(_ project: FlutterDartProject) {
+        dartProject = project
+    }
+
     /// Sets the plugin registrant callback for background task execution.
     ///
     /// This callback is used to register additional plugins when background tasks