fix(ci): use Flutter action instead of Dart in reusable workflow

The reusable workflow was failing for packages because melos bootstrap
needs Flutter to be available in the environment to bootstrap the entire
workspace, even when testing Dart-only packages.

Changed from dart-lang/setup-dart to subosito/flutter-action which
includes Dart and has built-in caching. This fixes the error:
/bin/sh: 1: eval: flutter: not found

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: grdsdev

.github/workflows/dart-package-test.yml

@@ -51,21 +51,11 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      - name: Setup Dart
-        uses: dart-lang/setup-dart@v1
+      - name: Setup Flutter (includes Dart)
+        uses: subosito/flutter-action@v2
         with:
-          sdk: ${{ matrix.sdk }}
-
-      - name: Cache pub dependencies
-        uses: actions/cache@v4
-        with:
-          path: |
-            ${{ env.PUB_CACHE }}
-            ~/.pub-cache
-          key: ${{ runner.os }}-pub-${{ matrix.sdk }}-${{ hashFiles('**/pubspec.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-pub-${{ matrix.sdk }}-
-            ${{ runner.os }}-pub-
+          channel: ${{ matrix.sdk }}
+          cache: true
 
       - name: Bootstrap workspace
         run: |