Fix corner-cases in ConeTypeContext / FIR builder / FIR enhancements

This fixes a pack of FIR smoke diagnostic tests, now all of them pass
Related to KT-29962

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/enhancement/javaTypeUtils.kt

@@ -146,7 +146,9 @@ private fun JavaClassifierType.enhanceInflexibleType(
                     mappedId = mappedId.readOnlyToMutable() ?: mappedId
                 }
             }
-            session.service<FirSymbolProvider>().getClassLikeSymbolByFqName(mappedId ?: classId)!!
+            val kotlinClassId = mappedId ?: classId
+            session.service<FirSymbolProvider>().getClassLikeSymbolByFqName(kotlinClassId)
+                ?: return ConeClassErrorType("Cannot find class-like symbol for $kotlinClassId during enhancement")
         }
         is JavaTypeParameter -> createTypeParameterSymbol(session, classifier.name)
         else -> return toNotNullConeKotlinType(session)

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/scopes/JavaClassEnhancementScope.kt

@@ -198,6 +198,7 @@ class JavaClassEnhancementScope(
     }
 
     private fun StringBuilder.appendConeType(coneType: ConeKotlinType) {
+        if (coneType is ConeClassErrorType) return
         append("L")
         when (coneType) {
             is ConeClassLikeType -> {