Add support for IntelliJ Platform 2025.3

- Update JVM target to 21 (required for IntelliJ 2024.2+)
- Add unified IntelliJ IDEA distribution support for 2025.3
  Starting from 2025.3, Community and Ultimate are merged into one
- Configure gradle-253.properties with 2025.3 IDE versions

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: meanmail

buildSrc/src/main/kotlin/common-conventions.gradle.kts

@@ -1,4 +1,4 @@
-import org.gradle.api.JavaVersion.VERSION_17
+import org.gradle.api.JavaVersion.VERSION_21
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
@@ -22,8 +22,9 @@ java {
     }
   }
 
-  sourceCompatibility = VERSION_17
-  targetCompatibility = VERSION_17
+  // IntelliJ Platform 2024.2+ requires Java 21
+  sourceCompatibility = VERSION_21
+  targetCompatibility = VERSION_21
 }
 
 kotlin {
@@ -54,7 +55,8 @@ tasks {
   }
   withType<KotlinCompile> {
     compilerOptions {
-      jvmTarget = JvmTarget.JVM_17
+      // IntelliJ Platform 2024.2+ requires Java 21
+      jvmTarget = JvmTarget.JVM_21
       languageVersion = KotlinVersion.DEFAULT
       // see https://plugins.jetbrains.com/docs/intellij/using-kotlin.html#kotlin-standard-library
       apiVersion = KotlinVersion.KOTLIN_1_8

buildSrc/src/main/kotlin/intellijUtils.kt

@@ -131,7 +131,18 @@ fun String.toTypeWithVersion(): TypeWithVersion {
 
 fun IntelliJPlatformDependenciesExtension.intellijIde(versionWithCode: String) {
   val (type, version) = versionWithCode.toTypeWithVersion()
-  create(type, version, useInstaller = false)
+  // Starting from 2025.3, IntelliJ IDEA Community and Ultimate are unified into a single distribution.
+  // Use the new intellijIdea() helper which resolves the unified artifact.
+  // See: https://blog.jetbrains.com/platform/2025/11/intellij-platform-2025-3-what-plugin-developers-should-know/
+  if (type == IntelliJPlatformType.IntellijIdeaUltimate && version.startsWith("2025.3")) {
+    intellijIdea(version) {
+      useInstaller.set(false)
+    }
+  } else {
+    create(type, version) {
+      useInstaller.set(false)
+    }
+  }
   // JetBrains runtime is necessary not only for running IDE but for tests as well
   jetbrainsRuntime()
 }

gradle-253.properties

@@ -0,0 +1,10 @@
+customSinceBuild=253
+customUntilBuild=253.*
+
+# Existent IDE versions can be found in the following repos:
+# https://www.jetbrains.com/intellij-repository/releases/
+# https://www.jetbrains.com/intellij-repository/snapshots/
+ideaVersion=IU-2025.3
+clionVersion=CL-2025.3
+pycharmVersion=PC-2025.3
+riderVersion=RD-2025.3