ci: split JVM and iOS test jobs, bump ubuntu to JDK 21 (#517)

Previously both ubuntu and macos runners ran the full JVM + Android
test suite, with macos additionally running iOS simulator tests. Mac
runners are ~10x the cost of Linux on GH Actions and the JVM/Android
duplication isn't earning much — KMP JVM bytecode behavior is nearly
identical across OSes.

I split this into two parallel jobs: ubuntu runs ktlint + JVM/Android,
macos only runs iOS simulator tests. Both now use JDK 21.

Note: branch protection rules referencing the old check names
(`Tests (ubuntu-latest, JDK 17)` / `Tests (macos-latest, JDK 21)`)
will need updating to the new names.

Co-authored-by: Miley Chandonnet <miley@Mileys-Mac-mini.local>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

.github/workflows/ci.yml

@@ -7,28 +7,18 @@ on:
     branches: [ main ]
 
 jobs:
-  tests:
-    name: Tests (${{ matrix.os }}, JDK ${{ matrix.java }})
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - os: ubuntu-latest
-            java: '17'
-            ios: false
-          - os: macos-latest
-            java: '21'
-            ios: true
+  jvm-tests:
+    name: JVM + Android Tests (ubuntu, JDK 21)
+    runs-on: ubuntu-latest
 
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
 
-      - name: Set up JDK ${{ matrix.java }}
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
-          java-version: ${{ matrix.java }}
+          java-version: '21'
           distribution: 'temurin'
 
       - name: Setup Gradle
@@ -50,7 +40,31 @@ jobs:
         timeout-minutes: 20
         run: ./gradlew :ampere-core:jvmTest :ampere-core:testDebugUnitTest :ampere-cli:jvmTest :ampere-compose:jvmTest :ampere-compose:testDebugUnitTest
 
+  ios-tests:
+    name: iOS Simulator Tests (macos, JDK 21)
+    runs-on: macos-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 21
+        uses: actions/setup-java@v4
+        with:
+          java-version: '21'
+          distribution: 'temurin'
+
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Create local.properties
+        run: |
+          cat > local.properties << 'EOF'
+          anthropic_api_key=placeholder
+          google_api_key=placeholder
+          openai_api_key=placeholder
+          EOF
+
       - name: Run iOS simulator tests
-        if: ${{ matrix.ios }}
         timeout-minutes: 25
         run: ./gradlew :ampere-core:iosSimulatorArm64Test