Model variants aren't working as FastAPI `response_model`

[vpetrovykh]

Describe the bug Using a model derived from a __variant__ as response_model causes object has no attribute '__gel_changed_fields__' PydanticSerializationError.

Reproduction

1. create a new venv
2. pip install "fastapi[standard]"
3. for the gel-python master branch pip install -Uve ".[test]"
4. gel init
5. Use this schema:

   module default {
     type User {
       required name: str {
         constraint exclusive;
       }
       multi friends: User;
     }
   }
   

6. gel migration create
7. gel migrate
8. gel query 'insert User{name := "Alice"}'
9. gel generate py/models (this is so FastAPI code has models to import)
10. create main.py

    import fastapi
    import gel.fastapi
    import uuid
    
    from pydantic import BaseModel
    from models import default
    
    app = fastapi.FastAPI()
    g = gel.fastapi.gelify(app)
    
    
    class MyUser(BaseModel):
        id: uuid.UUID
        name: str
        email: str
    
    
    class BaseUser(default.User.__variants__.Base):
        name: default.User.__typeof__.name
        email: default.User.__typeof__.email
    
    
    @app.get("/myuser/", response_model=list[MyUser])
    async def get_users(name: str):
        db = g.client
        q = default.User.filter(
            name=name,
        )
    
        return await db.query(q)
    
    
    @app.get("/baseuser/", response_model=list[BaseUser])
    async def broken_get_users(name: str):
        db = g.client
        q = default.User.filter(
            name=name,
        )
    
        return await db.query(q)
    

Try these 2 endpoints:

• curl -X 'GET' 'http://127.0.0.1:8000/myuser/?name=Alice' -H 'accept: application/json'
• curl -X 'GET' 'http://127.0.0.1:8000/baseuser/?name=Alice' -H 'accept: application/json'

The baseuser one should cause an error:

...
pydantic_core._pydantic_core.PydanticSerializationError: Error calling function `<lambda>`: AttributeError: 'BaseUser' object has no attribute '__gel_changed_fields__'

Expected behavior There should not be any difference in how these endpoints work.

Versions (please complete the following information):

• OS: Ubuntu
• EdgeDB version: Gel 6.9+77bc1f3
• EdgeDB CLI version: repl 7.7.0+3788f53
• gel-python version: master (c7aea17d877961ae81b1e661d66746aadab1ffff)
• Python version: 3.12.9