Fix regression in dataclass narrowing for Python >= 3.13

[ygale]

Fixes #21635.

What's going wrong

On Python >= 3.13, @dataclass synthesizes a __replace__(self, ...) -> Self
method to support copy.replace(). When mypy tries to narrow a type[A]
expression via issubclass(cls, M) (or isinstance) against a second,
unrelated dataclass M, it builds an ad-hoc <subclass of A and M> type to
check whether that narrowing is sound (intersect_instances in
checker.py). Building that ad-hoc type runs check_multiple_inheritance,
which sees A's synthesized __replace__ returning A and M's
returning M, and flags them as incompatible.

That's a false positive: a real subclass of both (e.g. class C(M, A),
itself decorated with @dataclass) gets its own freshly synthesized,
mutually compatible __replace__.

The result

In an if statement where there should have been type narrowing, the type resolves as Never, and so the block is ignored by the type checker. Type checking always silently succeeds without actually checking.

In a comprehension, the if clause is assigned the type Never and so is ignored by the type checker. The object whose type should have been narrowed retains its original type while type checking the comprehension expression.

The fix

check_compatibility already exempts __init__, __new__, and
__init_subclass__ from this cross-base check, because those are expected
to vary safely per concrete subclass. __replace__ has the same character
when it's plugin-generated (Self-returning, regenerated fresh per
concrete class), so this adds it to that exemption -- but only when the
method is plugin_generated on both sides, so hand-written __replace__
overrides with genuine incompatibilities are still caught.

Not included in this PR

• The TypeIs failure mentioned in the description of Dataclasses fail to narrow with issubclass() in Python >= 3.13 #21635 turns out to be a completely different failure mode. I will open a separate issue for that.
• There is a similar failure for NamedTuple, but that also turns out to be a completely different failure mode. I will also open a separate issue for that.

Tests

• New test cases in check-dataclasses.test with the reproduction from the description of Dataclasses fail to narrow with issubclass() in Python >= 3.13 #21635
• New test cases in check-dataclasses.test with confirming that hand-written incompatible
  __replace__ methods are still flagged.
• Ran the full dataclass/isinstance/narrow/typeguard/comprehension/
  multiple-inheritance test suites locally; all pass.
• black, ruff check, and codespell all clean on the changed file.
• mypy's own self-check on checker.py is clean.

LLM Disclosure

I was assisted by Claude (Sonnet 5), under my close guidance and supervision. The work is mine.

[github-actions]

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

steam.py (https://github.com/Gobot1234/steam.py)
- steam/profile.py:450: error: Definition of "__replace__" in base class "ProfileInfo" is incompatible with definition in base class "EquippedProfileItems"  [misc]
- steam/profile.py:461: error: Definition of "__replace__" in base class "ProfileInfo" is incompatible with definition in base class "EquippedProfileItems"  [misc]

dd-trace-py (https://github.com/DataDog/dd-trace-py)
- ddtrace/debugging/_signal/model.py:92: error: Subclass of "Probe" and "TimingMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_signal/model.py:93: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_signal/model.py:96: error: Cannot determine type of "_timing"  [has-type]
- ddtrace/debugging/_signal/model.py:101: error: Subclass of "Probe" and "ProbeConditionMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_signal/model.py:105: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_signal/model.py:127: error: Subclass of "Probe" and "RateLimitMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_signal/model.py:131: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_signal/model.py:182: error: Cannot determine type of "_timing"  [has-type]
- ddtrace/debugging/_signal/model.py:215: error: Cannot determine type of "_timing"  [has-type]
- ddtrace/debugging/_signal/log.py:41: error: Subclass of "Probe" and "LineLocationMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_signal/log.py:42: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_signal/log.py:46: error: Subclass of "Probe" and "FunctionLocationMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_signal/log.py:47: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_signal/log.py:54: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_signal/snapshot.py:204: error: Need type annotation for "captures" (hint: "captures: dict[<type>, <type>] = ...")  [var-annotated]
- ddtrace/debugging/_signal/snapshot.py:205: error: Subclass of "Probe" and "LogProbeMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_signal/snapshot.py:205: error: Right operand of "and" is never evaluated  [unreachable]
- ddtrace/debugging/_signal/snapshot.py:206: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_probe/registry.py:47: error: Subclass of "Probe" and "ProbeLocationMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_probe/registry.py:48: error: Statement is unreachable  [unreachable]
+ ddtrace/debugging/_debugger.py:536: error: Redundant cast to "FunctionType"  [redundant-cast]
- ddtrace/debugging/_debugger.py:374: error: Subclass of "Probe" and "LineLocationMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_debugger.py:376: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_debugger.py:500: error: Subclass of "Probe" and "FunctionLocationMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_debugger.py:503: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_debugger.py:619: error: Subclass of "Probe" and "LineLocationMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_debugger.py:620: error: Statement is unreachable  [unreachable]
- ddtrace/debugging/_debugger.py:621: error: Subclass of "Probe" and "FunctionLocationMixin" cannot exist: would have incompatible method signatures  [unreachable]
- ddtrace/debugging/_debugger.py:622: error: Statement is unreachable  [unreachable]