Dependencies - upgrade IntelliJ IDEA to 2026.2.3

Raise the default platform baseline to 262 and document its Java 25 toolchain, while keeping Java 21 for launching Gradle. Add the maintainer-recommended Ultimate test dependency for the classpath conflict reported in https://github.com/JetBrains/intellij-platform-gradle-plugin/issues/2261. All three tests, packaging, project configuration, and Plugin Verifier pass with released dependencies.

Co-authored-by: Junie <junie@jetbrains.com>

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 ### Changed
 
+- Dependencies - upgrade IntelliJ IDEA to `2026.2.3` (minimum platform `262`, Java `25`)
 - Dependencies - upgrade `org.jetbrains.kotlin.jvm` to `2.4.21`
 - Upgrade Gradle Wrapper to `9.8.1`
 - Update `intellijIdea` to `2025.3.6.1`

README.md

@@ -99,6 +99,9 @@ The most significant parts of the current configuration are:
 
 For more details regarding Kotlin integration, please see [Kotlin for Plugin Developers][docs:kotlin] in the IntelliJ Platform Plugin SDK documentation.
 
+The current target is IntelliJ IDEA 2026.2, with minimum platform build `262` and Java 25 required for compilation and testing.
+The IntelliJ Platform Gradle Plugin provisions the matching toolchain; Gradle itself can still run on Java 21, as it does in CI.
+
 ### Gradle properties
 
 The project-specific configuration file [`gradle.properties`][file:gradle.properties] starts with values expected to vary between repositories created from this template:

build.gradle.kts

@@ -11,7 +11,8 @@ dependencies {
 
     // IntelliJ Platform Gradle Plugin Dependencies Extension - read more: https://plugins.jetbrains.com/docs/intellij/tools-intellij-platform-gradle-plugin-dependencies-extension.html
     intellijPlatform {
-        intellijIdea("2025.3.6.1")
+        intellijIdea("2026.2.3")
         testFramework(TestFrameworkType.Platform)
+        testBundledPlugins("com.intellij.modules.ultimate")
     }
 }