OpenAPI spec compliance

Ensure all API definitions strictly follow OpenAPI specification standards to maintain compatibility with tooling and clients. This includes:

1. Match default values with their defined types:

   "data": {
     "type": "object",
     "description": "Document data as JSON object.",
   -  "default": [],
   +  "default": {},
   }
   

2. Place custom properties under vendor extensions (x-prefix):

   - "methods": [
   + "x-appwrite-methods": [
     {
    "name": "createDocument",
    ...
     }
   ]
   

3. Use proper variable syntax for URL templates:

   - "url": "https://<REGION>.cloud.appwrite.io/v1"
   + "url": "https://{region}.cloud.appwrite.io/v1",
   + "variables": {
   +   "region": {
   +     "description": "Regional subdomain (e.g., us-east-1)",
   +     "default": "us-east-1"
   +   }
   + }
   

Maintaining specification compliance ensures API documentation is accurate, enables proper code generation, and provides a consistent developer experience.