Change default IdeFlavor fallback from IC to IU for 2025.3

- Update toolkit-intellij-plugin.gradle.kts default from IC to IU
- This catches any modules that don't explicitly set their flavor
- Fixes remaining 'No IntelliJ Platform dependency found with IC-2025.3' errors

Author: aseemxs

buildSrc/src/main/kotlin/toolkit-intellij-plugin.gradle.kts

@@ -21,7 +21,8 @@ when {
     }
 
     else -> {
-        intellijToolkit.ideFlavor.set(IdeFlavor.IC)
+        // For 2025.3+, use IU as default since IC (Community) was discontinued
+        intellijToolkit.ideFlavor.set(IdeFlavor.IU)
     }
 }