Replace null checks with exceptions for DateTime types

Address PR feedback: throw InvalidOperationException when System.DateTime or System.DateTimeOffset types are not found in compilation, as these fundamental .NET types should always be available.

Co-authored-by: BenjaminMichaelis <22186029+BenjaminMichaelis@users.noreply.github.com>

Author: Copilot

IntelliTect.Analyzer/IntelliTect.Analyzer/Analyzers/BanImplicitDateTimeToDateTimeOffsetConversion.cs

@@ -49,13 +49,10 @@ private void AnalyzeConversion(OperationAnalysisContext context)
                 return;
             }
 
-            INamedTypeSymbol dateTimeType = context.Compilation.GetTypeByMetadataName("System.DateTime");
-            INamedTypeSymbol dateTimeOffsetType = context.Compilation.GetTypeByMetadataName("System.DateTimeOffset");
-
-            if (dateTimeType is null || dateTimeOffsetType is null)
-            {
-                return;
-            }
+            INamedTypeSymbol dateTimeType = context.Compilation.GetTypeByMetadataName("System.DateTime")
+                ?? throw new InvalidOperationException("System.DateTime type not found in compilation.");
+            INamedTypeSymbol dateTimeOffsetType = context.Compilation.GetTypeByMetadataName("System.DateTimeOffset")
+                ?? throw new InvalidOperationException("System.DateTimeOffset type not found in compilation.");
 
             // Check if source is DateTime and target is DateTimeOffset
             if (SymbolEqualityComparer.Default.Equals(sourceType, dateTimeType) && 
@@ -73,13 +70,10 @@ private void AnalyzeBinaryOperation(OperationAnalysisContext context)
                 return;
             }
 
-            INamedTypeSymbol dateTimeType = context.Compilation.GetTypeByMetadataName("System.DateTime");
-            INamedTypeSymbol dateTimeOffsetType = context.Compilation.GetTypeByMetadataName("System.DateTimeOffset");
-
-            if (dateTimeType is null || dateTimeOffsetType is null)
-            {
-                return;
-            }
+            INamedTypeSymbol dateTimeType = context.Compilation.GetTypeByMetadataName("System.DateTime")
+                ?? throw new InvalidOperationException("System.DateTime type not found in compilation.");
+            INamedTypeSymbol dateTimeOffsetType = context.Compilation.GetTypeByMetadataName("System.DateTimeOffset")
+                ?? throw new InvalidOperationException("System.DateTimeOffset type not found in compilation.");
 
             // For binary operations, check if either operand is DateTime and the other is DateTimeOffset
             // This catches cases where IConversionOperation nodes are not created (e.g., property access)