fix(kotlin-ls): improve root detection for Gradle multi-project builds

[JBou]

Ensure the Kotlin language server is started at the correct Gradle root for multi-project and composite builds.

The root resolution now prioritizes settings.gradle(.kts), followed by the Gradle wrapper, before falling back to module-level build files or Maven projects. This prevents submodules from being incorrectly detected as the workspace root.

Follow up for #6601