gh-148941: Skip generating __init__, __repr__, __eq__ when class already defines them

[q121212]

gh-148941: Skip generating init, repr, eq when class already defines them

Bug

When @dataclass(init=True) (the default) is applied to a class that
already defines __init__ in its own __dict__, the generated
__init__ is always discarded by _set_new_attribute. However,
_init_fn was still called unconditionally, and its field-ordering
validation raised TypeError when inherited fields with defaults were
followed by fields without defaults:

from dataclasses import dataclass

@dataclass
class Base:
    x: int = 0  # has a default

@dataclass
class Child(Base):
    y: int      # no default — would make generated signature invalid
    def __init__(self, y):
        self.y = y
# TypeError: non-default argument 'y' follows default argument 'x'

The docs state: "If the class already defines __init__(), this
parameter is ignored." The design tables at the top of
Lib/dataclasses.py also document this as the intended behaviour:
for all of __init__, __repr__, and __eq__, the cell where
parameter=True and class has method in __dict__ is intentionally
blank (= no action).

Fix

Add '__x__' not in cls.__dict__ guards to the if init:, if repr:,
and if eq: blocks in _process_class. This skips the generation
(and its validation) entirely when the class's own method would have
been preserved by _set_new_attribute anyway.

Methods with 'raise' cells in the design tables (order, frozen)
are intentionally NOT skipped early — their existing
TypeError-on-overwrite behaviour is correct.

Tests

test_skip_methods_when_defined.py added to Lib/test/test_dataclasses/
covering:

• __init__: base with default + child without default (the exact
  reproducer from dataclass tries to generate __init__() even though the class already defines one #148941), default_factory variant, KW_ONLY variant,
  three-level inheritance chain
• __init__: NOT skipped when only a base class defines it (only
  cls.__dict__ matters)
• __repr__: skipped when custom __repr__ defined
• __eq__: skipped when custom __eq__ defined
• order / frozen: still raise TypeError (raise cells untouched)

All existing dataclasses tests continue to pass.

NEWS

Misc/NEWS.d/next/Library/...rst added.

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