fix: update fjs_type validation to check for toJSON method

Author: 123NeNaD

lib/validator.js

@@ -21,8 +21,8 @@ class Validator {
       keyword: 'fjs_type',
       type: 'object',
       errors: false,
-      validate: (_type, date) => {
-        return date instanceof Date
+      validate: (_type, data) => {
+        return data && typeof data.toJSON === 'function'
       }
     })
 
@@ -51,8 +51,10 @@ class Validator {
     return this.ajv.validate(schemaRef, data)
   }
 
-  // Ajv does not support js date format. In order to properly validate objects containing a date,
-  // it needs to replace all occurrences of the string date format with a custom keyword fjs_type.
+  // Ajv does not natively support JavaScript objects like Date or other types
+  // that rely on a custom .toJSON() representation. To properly validate schemas
+  // that may contain such objects (e.g. Date, ObjectId, etc.), we replace all
+  // occurrences of the string type with a custom keyword fjs_type
   // (see https://github.com/fastify/fast-json-stringify/pull/441)
   convertSchemaToAjvFormat (schema) {
     if (schema === null) return