Refine OpenAPI schema and enforce strict validation for IDs

[itallix]

Currently, the OpenAPI schema is shared across all APIs for simplicity. This leads to inconsistent validation requirements (the id field being optional). The schema should be split properly so each API (GET, PATCH, POST) can have its own validation rules.

Refine the OpenAPI schema to enforce strict validation for the id field where required, so that the UI does not need to implement conditional logic for handling missing IDs.

For creation requests, the id field can remain optional.