Fix Test workflow compile failure by restoring IntelliJ platform classpath in Gradle 2.x migration

[Copilot]

The failing Test job was caused by a broken IntelliJ Platform dependency declaration after migrating to org.jetbrains.intellij.platform (Gradle 2.x API). As a result, compileKotlin ran without IntelliJ SDK classes and failed with broad unresolved symbol errors.

• Build config: correct platform dependency wiring

  • Updated build.gradle.kts to map configured platform type to explicit IntelliJ dependency helpers:
    • IU → intellijIdeaUltimate(platformVersion)
    • IC → intellijIdeaCommunity(platformVersion)
    • fallback retains create(platformType, platformVersion)
  • Kept plugin dependency resolution logic unchanged.

• Why this matters

  • Ensures the IntelliJ Platform artifact is attached to the compilation classpath under the new plugin API, eliminating the missing IntelliJ/Kotlin type cascade in CI.

intellijPlatform {
    when (platformType) {
        "IU" -> intellijIdeaUltimate(platformVersion)
        "IC" -> intellijIdeaCommunity(platformVersion)
        else -> create(platformType, platformVersion)
    }
    testFramework(TestFrameworkType.Platform)
}