fix: correct allows_all containment check for "!=" vs "not in"

[hexonal]

Fixes python-poetry/poetry#10977

Constraint.allows_all() had a containment check backwards for the case where self is a != constraint and other is a not in constraint. It tested self.value not in other.value, but the correct check has the same shape as the existing not in / not in branch just above it: other.value in self.value.

Concretely:

>>> from poetry.core.version.markers import parse_marker
>>> m = parse_marker('os_name != "a" and "b" not in os_name')
>>> str(m)  # before: the `!=` clause silently disappears
'"b" not in os_name'
>>> m.validate({"os_name": "a"})  # before: True (wrong)
>>> m.validate({"os_name": "a"})  # after: False (correct)

This happened because MultiMarker/constraint intersection logic uses allows_all to drop a clause it believes is redundant. The old formula wrongly considered os_name != "a" redundant given "b" not in os_name, even though os_name == "a" satisfies the latter but not the former.

I merged the two branches for self._operator == "not in" and self._operator == "!=" under other.operator == "not in" since they resolve to the identical correct formula.

Added regression tests in tests/constraints/generic/test_constraint.py (direct allows_all cases) and tests/version/test_markers.py (the exact marker/validate scenario from the issue, plus two adjacent cases).

[sourcery-ai]

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