Add idea 2026.1 to GitHub workflow

[hduelme]

I added IDEA2026.1 to the GitHub workflow.
For this I upgraded the org.jetbrains.intellij.platform plugin to 2.15.0. This makes the workaround for older versions of idea obsolete.
In addition I replaced testFramework( TestFrameworkType.Bundled.INSTANCE ) (should not be used see https://plugins.jetbrains.com/docs/intellij/tools-intellij-platform-gradle-plugin-types.html#TestFrameworkType-Platform) with testFramework(TestFrameworkType.Plugin.Java.INSTANCE). As a reference I used github.com/JetBrains/intellij-sdk-code-samples/blob/main/simple_language_plugin/build.gradle.kts

[filiphr]

Thanks @hduelme. This looks clean. I've left one small comment. By the way looking now at all the versions we test with, I think we need to drop 2024 and use 2025.1 as a minimum.

[hduelme]

@filiphr good catch. I removed the duplicated version.
I agree currently we test against lost of versions. As you suggested, I upgraded sinceBuild to idea 2025.1.

[filiphr]

Awesome. Thanks @hduelme