refactor: strengthen client boundaries and quality gates

api/src/common/bounded-json-object.validator.ts

@@ -0,0 +1,55 @@
+import { registerDecorator, type ValidationArguments, type ValidationOptions } from 'class-validator';
+
+type BoundedJsonOptions = {
+  maxDepth?: number;
+  maxKeys?: number;
+  maxStringLength?: number;
+};
+
+const FORBIDDEN_KEYS = new Set(['__proto__', 'constructor', 'prototype']);
+
+export function IsBoundedJsonObject(options: BoundedJsonOptions = {}, validationOptions?: ValidationOptions) {
+  return (target: object, propertyName: string) =>
+    registerDecorator({
+      name: 'isBoundedJsonObject',
+      target: target.constructor,
+      propertyName,
+      constraints: [options],
+      options: validationOptions,
+      validator: {
+        validate(value: unknown, args: ValidationArguments) {
+          if (value === undefined || value === null) return true;
+          const [constraints] = args.constraints as [BoundedJsonOptions];
+          return isBoundedJsonValue(value, {
+            maxDepth: constraints.maxDepth ?? 4,
+            maxKeys: constraints.maxKeys ?? 100,
+            maxStringLength: constraints.maxStringLength ?? 2_000,
+          });
+        },
+        defaultMessage(args: ValidationArguments) {
+          return `${args.property} contains too many, too deeply nested, or unsafe values`;
+        },
+      },
+    });
+}
+
+function isBoundedJsonValue(value: unknown, limits: Required<BoundedJsonOptions>) {
+  let keyCount = 0;
+  const visit = (current: unknown, depth: number): boolean => {
+    if (depth > limits.maxDepth) return false;
+    if (current == null || typeof current === 'boolean' || typeof current === 'number') return true;
+    if (typeof current === 'string') return current.length <= limits.maxStringLength;
+    if (Array.isArray(current)) {
+      keyCount += current.length;
+      return keyCount <= limits.maxKeys && current.every((item) => visit(item, depth + 1));
+    }
+    if (typeof current !== 'object') return false;
+    const entries = Object.entries(current as Record<string, unknown>);
+    keyCount += entries.length;
+    return (
+      keyCount <= limits.maxKeys &&
+      entries.every(([key, item]) => key.length <= 128 && !FORBIDDEN_KEYS.has(key) && visit(item, depth + 1))
+    );
+  };
+  return visit(value, 0);
+}