Fix crash on dataclass field assigned a functional namedtuple() call - #21849

Fixes #21583

Root cause

from collections import namedtuple
from dataclasses import dataclass

@dataclass
class C:
    p: "Point" = namedtuple("Point", ["x", "y"])

crashes mypy with:

AssertionError: TypeInfo(
  Name(crash.C.p)
  ...

When the right-hand side of a class-body assignment is a namedtuple(...)
call, mypy's semantic analyzer (analyze_namedtuple_assign in
mypy/semanal.py) always treats the statement as a functional NamedTuple
class definition and binds the assigned name in the class's symbol table to
a TypeInfo, regardless of whether the left-hand side also carries a type
annotation. So p's symbol table entry ends up being a TypeInfo for a
synthesized Point class, not a Var, even though it looks like a normal
annotated dataclass field.

DataclassTransformer.collect_attributes() in mypy/plugins/dataclasses.py
did not anticipate this and unconditionally asserted
isinstance(node, Var) for any class-body annotated assignment that wasn't
already special-cased (TypeAlias, Decorator), causing the crash.

Fix

Skip such fields the same way TypeAlias and Decorator nodes are already
skipped in collect_attributes(), since a name bound to a TypeInfo this
way is not a valid dataclass field. This mirrors the existing handling
immediately above it in the same function and does not touch the
Var-based path used by ordinary dataclass/NamedTuple fields.

Testing

• Added testNoCrashForDataclassFieldAssignedFunctionalNamedTuple to
  test-data/unit/check-dataclasses.test, covering both the
  name-mismatched case from the issue and a name-matched variant.
• python -m pytest -q mypy/test/testcheck.py -k dataclass — 204 passed.
• python -m pytest -q mypy/test/testcheck.py -k "namedtuple or NamedTuple" — 225 passed.
• Manually confirmed the exact repro from the issue no longer crashes, and
  that normal dataclass fields with forward-referenced NamedTuple
  class types still type-check correctly.
• pre-commit run --files mypy/plugins/dataclasses.py test-data/unit/check-dataclasses.test — all checks passed.