Report illegal codegen parameter types at declaration

[arpitagarwal1301]

Fixes #1084

Summary

• report illegal direct Unit, Nothing, and Void constructor parameters on the parameter element
• unwrap aliases and ignore nullability only for validation while preserving the original generated type
• avoid catch-all stack traces and misleading missing-constructor diagnostics

Validation

• focused KSP compile-testing regressions, including chained aliases and List<Unit>
• ./gradlew spotlessCheck :moshi-kotlin-codegen:check
• ./gradlew build check -PkotlinTestMode=KSP
• ./gradlew build check -PkotlinTestMode=REFLECT
• git diff --check