Gradle grid: JDK 15 ceiling from Zulu (Temurin never shipped 15)

setup-java has no Temurin 15, so the 6.9.4 ceiling rows take Zulu 15; the
CI-tier tests now pin the 6.9 <= 15 ceiling.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: mikolalysenko

.github/workflows/gradle-compatibility.yml

@@ -229,7 +229,8 @@ jobs:
         if: steps.jdk.outputs.runner-jdk != 'true'
         uses: actions/setup-java@de7274f081f381c8f8158605e0321c36c376e2e6 # v6.0.1
         with:
-          distribution: temurin
+          # Temurin never shipped JDK 15 (the 6.9 ceiling); Zulu did.
+          distribution: ${{ matrix.java == '15' && 'zulu' || 'temurin' }}
           java-version: ${{ matrix.java }}
 
       - name: Install Maven 3.9.16 (vendor cells)

scripts/tests/test_ci_e2e_tiers.py

@@ -227,7 +227,7 @@ def test_compat_grid_expands_to_36_cells_plus_extras(self):
             labels.setdefault(row["label"], []).append(row)
             self.assertEqual(row["os"], "ubuntu-latest")
         ceilings = {(r["gradle"], r["java"]) for r in labels["jdk-ceiling"]}
-        self.assertEqual(ceilings, {("6.9.4", "16"), ("7.6.6", "19"), ("8.14.3", "24")})
+        self.assertEqual(ceilings, {("6.9.4", "15"), ("7.6.6", "19"), ("8.14.3", "24")})
         self.assertEqual(len(labels["jdk-ceiling"]), 9)
         self.assertEqual({(r["gradle"], r["mode"]) for r in labels["configuration-cache"]},
                          {("9.8.0", "hosted"), ("9.8.0", "vendor")})
@@ -251,7 +251,7 @@ def test_compat_workflow_builds_its_own_binaries(self):
                      "crates/socket-patch-core/src/vendor/jvm/**", "crates/socket-patch-core/src/patch/jvm_jar.rs",
                      "crates/socket-patch-cli/tests/jvm_fixture_repo/**", "crates/socket-patch-cli/tests/e2e_*gradle*"):
             self.assertIn(f"'{path}'", text)
-        self.assertIn("6.9 <= 16", text)
+        self.assertIn("6.9 <= 15", text)
         self.assertIn("7.6 <= 19", text)
         self.assertIn("8.14 <= 24", text)
         self.assertIn("gradle-probe", text)