fix(csharp): count System.Text.Json string bounds in code points

packages/quicktype-core/src/language/CSharp/SystemTextJsonCSharpRenderer.ts

@@ -1141,16 +1141,21 @@ export class SystemTextJsonCSharpRenderer extends CSharpRenderer {
                 emitFinish,
             );
         } else if (xfer instanceof MinMaxLengthCheckTransformer) {
+            const length = [
+                "System.Linq.Enumerable.Count(",
+                variable,
+                "!, c => !char.IsLowSurrogate(c))",
+            ];
             const min = xfer.minLength;
             const max = xfer.maxLength;
             const conditions: Sourcelike[] = [];
 
             if (min !== undefined) {
-                conditions.push([variable, ".Length >= ", min.toString()]);
+                conditions.push([length, " >= ", min.toString()]);
             }
 
             if (max !== undefined) {
-                conditions.push([variable, ".Length <= ", max.toString()]);
+                conditions.push([length, " <= ", max.toString()]);
             }
 
             this.emitLine("if (", arrayIntercalate([" && "], conditions), ")");

test/languages.ts

@@ -186,12 +186,16 @@ export const CSharpLanguageSystemTextJson: Language = {
         "uuid",
         "minmax",
         "minmaxInteger",
+        "minmaxlength",
         "integer",
     ],
     output: "QuickType.cs",
     topLevel: "TopLevel",
     skipJSON: [],
     skipMiscJSON: false,
+    additionalSchemaFiles: [
+        "test/inputs/regressions/unicode-codepoint-length.schema",
+    ],
     skipSchema: [
         // The following skips are pre-existing System.Text.Json renderer issues,
         // found when first enabling the schema fixture for this language: