Describe the bug

I'm experimenting with the use of the MISSING sentinel added in pydantic 2.12. I ran into an issue during serialization where a default value could not be created for the sentinel which is triggered by the overriding of default_schema since the MISSING sentinel is not treated as a no-op.

I'm not sure the custom generator or method are needed, at least not for the allOf piece since I believe that changed in openapi schema v3.1, but I didn't trace all of that down. It may still be needed to handle how default factories and callables are handled.

Versions (please complete the following information):

• Python version: 3.12.12
• Django version: 5.28
• Django-Ninja version: 1.5.0
• Pydantic version: 2.12.4