[ty] Preserve type variables in bounded generic defaults (#28122)
A type parameter default such as `Box[T]` can incorrectly reject `T`
when `Box` has a bounded or constrained type parameter, leaving the
default specialized with `Unknown`. This change accepts compatible type
variables and preserves them so applying the default substitutes the
actual type argument.
Bind unresolved type variables in a temporary copy when validating
generic arguments against bounds or constraints. Keep the original type
in the specialization so defaults can still be bound to the generic that
uses them.
Fixes https://github.com/astral-sh/ty/issues/4408.
## Test plan
Added mdtests for defaults containing bounded and constrained type
variables in both PEP 695 and legacy generic syntax. The tests cover
class and type alias defaults, substitution through nested tuple
arguments, incompatible bounds and constraints, and rejection of
invariant list arguments.
