[ty] Restrict range implications from gradual equivalence (#29577)
Fix false `invalid-argument-type` errors when inferring constrained type
variables from callbacks that return an invariant generic with a gradual
type argument. A callback returning `Box[Any]` can satisfy `Callable[[],
T | Box[T]]` when `T` is constrained to `str` or `bytes`.
The solver treated equivalence to a gradual type as exact substitution,
allowing a valid union alternative to imply an incompatible bound.
Require a static equivalence bound before deriving either lower- or
upper-bound implications, including when gradual types are nested inside
a bound.
Prerequisite to avoid regressions in #29236.
