Add Kotlin DSL variable downgrade tests for UpgradePluginVersion

Add tests covering the scenario from customer-requests#1809 where
UpgradePluginVersion with an exact version could downgrade plugins
when versions are stored in a Kotlin DSL val variable shared across
multiple plugin declarations.

Also replace assert statements with proper null checks in the scanner
for the GString and Identifier variable branches to avoid silent null
storage when assertions are disabled.

Author: MBoegers

rewrite-gradle/src/main/java/org/openrewrite/gradle/plugins/UpgradePluginVersion.java

@@ -188,18 +188,20 @@ public J visitMethodInvocation(J.MethodInvocation method, ExecutionContext ctx)
                         String resolvedPluginVersion = new DependencyVersionSelector(metadataFailures, gradleProject, gradleSettings)
                                 .select(new GroupArtifact(pluginId, pluginId + ".gradle.plugin"), "classpath", newVersion, versionPattern, ctx);
 
-                        acc.versionPropNameToPluginId.put(versionVariableName, pluginId);
-                        assert resolvedPluginVersion != null;
-                        acc.pluginIdToNewVersion.put(pluginId, resolvedPluginVersion);
+                        if (resolvedPluginVersion != null) {
+                            acc.versionPropNameToPluginId.put(versionVariableName, pluginId);
+                            acc.pluginIdToNewVersion.put(pluginId, resolvedPluginVersion);
+                        }
                     } else if (versionArgs.get(0) instanceof J.Identifier) {
                         J.Identifier identifier = (J.Identifier) versionArgs.get(0);
                         String versionVariableName = identifier.getSimpleName();
                         String resolvedPluginVersion = new DependencyVersionSelector(metadataFailures, gradleProject, gradleSettings)
                                 .select(new GroupArtifact(pluginId, pluginId + ".gradle.plugin"), "classpath", newVersion, versionPattern, ctx);
 
-                        acc.versionPropNameToPluginId.put(versionVariableName, pluginId);
-                        assert resolvedPluginVersion != null;
-                        acc.pluginIdToNewVersion.put(pluginId, resolvedPluginVersion);
+                        if (resolvedPluginVersion != null) {
+                            acc.versionPropNameToPluginId.put(versionVariableName, pluginId);
+                            acc.pluginIdToNewVersion.put(pluginId, resolvedPluginVersion);
+                        }
                     }
                 } catch (MavenDownloadingException e) {
                     // continue

rewrite-gradle/src/test/java/org/openrewrite/gradle/plugins/UpgradePluginVersionTest.java

@@ -474,4 +474,59 @@ void doesNotPinPropertyManagedVersions() {
           )
         );
     }
+
+    @Test
+    void dontDowngradeKotlinDslSharedVariable() {
+        rewriteRun(
+          spec -> spec.recipe(new UpgradePluginVersion("org.jetbrains.kotlin.*", "1.5.31", null)),
+          buildGradleKts(
+            """
+              plugins {
+                  val kotlinVersion = "1.8.21"
+                  id("org.jetbrains.kotlin.jvm") version kotlinVersion
+                  id("org.jetbrains.kotlin.plugin.allopen") version kotlinVersion
+                  id("org.jetbrains.kotlin.plugin.spring") version kotlinVersion
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void dontDowngradeKotlinDslSinglePluginVariable() {
+        rewriteRun(
+          spec -> spec.recipe(new UpgradePluginVersion("org.jetbrains.kotlin.*", "1.5.31", null)),
+          buildGradleKts(
+            """
+              plugins {
+                  val kotlinVersion = "1.8.21"
+                  id("org.jetbrains.kotlin.jvm") version kotlinVersion
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void upgradeKotlinDslSharedVariable() {
+        rewriteRun(
+          spec -> spec.recipe(new UpgradePluginVersion("org.jetbrains.kotlin.*", "2.0.0", null)),
+          buildGradleKts(
+            """
+              plugins {
+                  val kotlinVersion = "1.9.24"
+                  id("org.jetbrains.kotlin.jvm") version kotlinVersion
+                  id("org.jetbrains.kotlin.plugin.allopen") version kotlinVersion
+              }
+              """,
+            """
+              plugins {
+                  val kotlinVersion = "2.0.0"
+                  id("org.jetbrains.kotlin.jvm") version kotlinVersion
+                  id("org.jetbrains.kotlin.plugin.allopen") version kotlinVersion
+              }
+              """
+          )
+        );
+    }
 }