fix: clean stale native asset state before iOS/macOS CI builds

The .dart_tool directory is cached by pubspec.lock hash, but it
contains native asset build metadata (NativeAssetsManifest.json)
that references compiled .dylibs in build/native_assets/ which
is not cached. When a cache hit restores stale .dart_tool state,
xcode_backend.dart fails trying to copy the missing objective_c
native asset. Clearing flutter_build and native_assets before
building forces a fresh native asset compilation.

Author: ericgriffin

.github/workflows/ci.yaml

@@ -236,6 +236,11 @@ jobs:
       - name: Update CocoaPods repo
         run: pod repo update
 
+      - name: Clean native asset build state
+        run: |
+          rm -rf build/native_assets
+          rm -rf .dart_tool/flutter_build
+
       - name: Build iOS
         run: flutter build ios --release --no-codesign "${{ env.DART_DEFINES }}" -v
 
@@ -315,6 +320,11 @@ jobs:
       - name: Update CocoaPods repo
         run: pod repo update
 
+      - name: Clean native asset build state
+        run: |
+          rm -rf build/native_assets
+          rm -rf .dart_tool/flutter_build
+
       - name: Build macOS (no-sandbox for distribution)
         run: |
           # Configure ad-hoc signing for CI (no developer certificate available)