Expand override manager create test + move to assert_type

[UnknownPlatypus]

I have made things!

Expand the coverage for maanger create overloads.
The unparametrized case exercise a bit of plugin but not the others two so I moved it to assert_type.

This also revealed that our typing of Model.objects as ClassVar[Manager[Self]] is already a bit problematic with pyright and pyrefly (see the type-ignore's). Given this exemple:

class Book(models.Model):
    objects = BookManager()
    foo = BookManager() 

I get these revealed types

┌─────────────┬───────────────┬─────────────┐
│             │ Book.objects  │  Book.foo   │
├─────────────┼───────────────┼─────────────┤
│ mypy+plugin │ BookManager   │ BookManager │
├─────────────┼───────────────┼─────────────┤
│ ty          │ BookManager   │ BookManager │
├─────────────┼───────────────┼─────────────┤
│ pyright     │ Manager[Book] │ BookManager │
├─────────────┼───────────────┼─────────────┤
│ pyrefly     │ Manager[Book] │ BookManager │
└─────────────┴───────────────┴─────────────┘

To me our declaration in the stubs should not impact regular class level assignment inference and I would expect objects to have the same type as foo. The plugin should only apply the "does not exist on abstract model" bit, the rest should be regular inference.

I'm exploring how to improve that, maybe with a descriptor in a followup pr

Related issues

Still trying to cut the bulk for #2776

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[sobolevn]

:(