Skip untyped internal C# RPC properties (#2298)

Remove internal properties without a representable schema shape before C# RPC generation, while preserving typed internal properties and strict failures for public schemas.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 510b87fe-e424-4389-a811-2a4c3a2181ff

Author: stephentoub

scripts/codegen/csharp.ts

@@ -355,6 +355,41 @@ function failUnmappable(context: string, schema: JSONSchema7): never {
     );
 }
 
+function omitUntypedInternalProperties(value: unknown): void {
+    if (!value || typeof value !== "object") return;
+    if (Array.isArray(value)) {
+        value.forEach(omitUntypedInternalProperties);
+        return;
+    }
+
+    const node = value as Record<string, unknown>;
+    const properties = node.properties;
+    if (properties && typeof properties === "object" && !Array.isArray(properties)) {
+        for (const [name, property] of Object.entries(properties)) {
+            if (!property || typeof property !== "object" || Array.isArray(property)) continue;
+            const schema = property as JSONSchema7;
+            const hasType =
+                schema.type !== undefined ||
+                schema.$ref !== undefined ||
+                schema.anyOf !== undefined ||
+                schema.oneOf !== undefined ||
+                schema.allOf !== undefined ||
+                schema.enum !== undefined ||
+                schema.const !== undefined ||
+                isOpaqueJson(schema);
+            if (isSchemaInternal(schema) && !hasType) {
+                delete (properties as Record<string, unknown>)[name];
+            } else {
+                omitUntypedInternalProperties(property);
+            }
+        }
+    }
+
+    for (const [name, child] of Object.entries(node)) {
+        if (name !== "properties") omitUntypedInternalProperties(child);
+    }
+}
+
 function requiresArgumentNullCheck(typeName: string, isRequired: boolean): boolean {
     return isRequired && !typeName.endsWith("?") && !isNonNullableCSharpValueType(typeName);
 }
@@ -2568,6 +2603,8 @@ function generateRpcCode(
     externalJsonSerializableRefs: Map<string, Set<string>> = new Map(),
     externalValueTypes: Set<string> = new Set()
 ): string {
+    schema = cloneSchemaForCodegen(schema);
+    omitUntypedInternalProperties(schema);
     emittedRpcClassSchemas.clear();
     emittedRpcEnumResultTypes.clear();
     experimentalRpcTypes.clear();