Update C# Views error messages with accurate return types

Author: rekhoff

crates/bindings-csharp/Codegen/Diag.cs

@@ -215,9 +215,9 @@ string typeName
     public static readonly ErrorDescriptor<MethodDeclarationSyntax> ViewInvalidReturn =
         new(
             group,
-            "Views must return Vec<T> or Option<T>",
-            method => $"View '{method.Identifier}' must return Vec<T> or Option<T>.",
-            method => method
+            "Views must return List<T> or T?",
+            method => $"View '{method.Identifier}' must return List<T> or T?.",
+            method => method.ReturnType
         );
 
     // TODO: Remove once Views support Private: Views must be Public currently

crates/bindings-csharp/Codegen/Module.cs

@@ -1189,7 +1189,7 @@ method.ReturnType is INamedTypeSymbol
                 ? "SpacetimeDB.BSATN.ValueOption"
                 : "SpacetimeDB.BSATN.RefOption";
             var opt = $"{optType}<{rowType.Name}, {rowType.BSATNName}>";
-            // Match Rust semantics: Query<T> is described as Option<T>.
+            // Match Rust semantics: Query<T> is described as a nullable row (T?).
             ReturnType = new ReferenceUse(opt, opt);
         }
         else
@@ -1210,7 +1210,7 @@ method.ReturnType is INamedTypeSymbol
             diag.Report(ErrorDescriptor.ViewContextParam, methodSyntax);
         }
 
-        // Validate return type: must be Option<T> or Vec<T>
+        // Validate return type: must be List<T> or T?
         if (
             !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.ValueOption")
             && !ReturnType.BSATNName.Contains("SpacetimeDB.BSATN.RefOption")