Add request payload validation middleware for /graphql and other endpoints

[mattfsourcecode]

Details:

• Use a validation library (e.g., joi or zod) to validate incoming request payloads.
• Prevent invalid data from reaching the business logic layer, reducing potential errors and improving API security.
• Define validation schemas for key fields such as headers, query parameters, and request bodies.

Action Items:

1. Create validation schemas for frequently used API endpoints (e.g., /graphql).
2. Integrate the validation middleware before the route handlers.
3. Write tests to verify that invalid payloads return appropriate error responses (e.g., 400 Bad Request).