Fix self type check for generic NamedTuple with constrained typevar

[EmmanuelNiyonshuti]

Fixes #21453

This PR makes it so that, for generic NamedTuples with a constrained typevar
and an unannotated self, the signature's self type with the substituted typevar
instance is used to substitute the ref_type(the class type seen from inside
the method), and they are the same type.
Otherwise the same-type check was failing, since ref_type would stay a typevar
type.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy-stubs (https://github.com/scipy/scipy-stubs)
+ tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

[chrikrah]

@EmmanuelNiyonshuti approving at f7037aa. Expanding ref_type through the self argument's partial_fallback makes the two sides comparable, and nothing else in the checks I ran moves.

Head f7037aa, merge base d22bde6, Python 3.12:

$ python -m pytest -n 4 -q mypy/test/testcheck.py::TypeCheckSuite::check-{namedtuple,selftype,classes,generics,tuples}.test
1141 passed, 2 skipped, 1 xfailed

# same command, checker.py from d22bde6, your test kept
FAILED ...check-namedtuple.test::testGenericNamedTupleWithConstrainedTypeParams
1 failed, 1140 passed, 2 skipped, 1 xfailed

non-blocking: the change also reaches an explicit narrowed self on any generic NamedTuple, constrained or not. Box below, a plain Generic[T] class, already passes on both revisions, and the call-site check still fires:

$ cat -n nt3.py
     1	from typing import Generic, NamedTuple, TypeVar
     2	
     3	T = TypeVar("T")
     4	
     5	class Plain(NamedTuple, Generic[T]):
     6	    value: T
     7	    def only_int(self: "Plain[int]") -> int:
     8	        return self.value
     9	
    10	class Box(Generic[T]):
    11	    value: T
    12	    def only_int(self: "Box[int]") -> int:
    13	        return self.value
    14	
    15	Plain(1).only_int()
    16	Plain("a").only_int()

$ python -m mypy nt3.py   # d22bde6
nt3.py:7: error: The erased type of self "tuple[int, fallback=nt3.Plain[int]]" is not a supertype of its class "tuple[T, fallback=nt3.Plain[T]]"  [misc]
nt3.py:16: error: Invalid self argument "Plain[str]" to attribute function "only_int" with type "Callable[[Plain[int]], int]"  [misc]
$ python -m mypy nt3.py   # f7037aa
nt3.py:16: error: Invalid self argument "Plain[str]" to attribute function "only_int" with type "Callable[[Plain[int]], int]"  [misc]

A second test case for the narrowed self would pin that behaviour.

mypy_self_check.ini on mypy/checker.py, black 26.1.0 and ruff 0.14.3 are clean.

@ilevkivskyi, should the narrowed-self case go in as a second test here, or stay out of this fix?