[ty] Constrain inferred constructor results using the initializer receiver - #30270

Summary

If Reader.__init__ annotates self as Reader[bool | None], assigning Reader() to a variable of type Reader[bool] can silently drop None.

Alternative to #30179 for astral-sh/ty#4977. This approach keeps the constructor result generic during contextual inference. The receiver argument already requires Reader[T] to be assignable to Reader[bool | None]. When inferring a bare constructor against an expected type, also require Reader[bool | None] to be assignable to the inferred result Reader[T]. For a covariant reader this supplies the missing lower bound on T, so the assignment reports invalid-assignment.

For an inherited initializer, first express the annotated result as a specialization of the subclass. Type arguments that the initializer leaves open can still come from the assignment. This also preserves the existing inference of X[Any] for x: X[Any] = X() with self: X[None].

Test plan

Regression cases cover mixed-variance fields in both generic syntaxes, explicit specialization, and inherited initializers.