Do not narrow Flag enums by member equality

[istoolsfox]

Fixes #21937

Description

narrow_type_by_identity_equality (used for ==/!=/is/is not narrowing) expands an enum-typed expression into a union of member literals and, in the negative branch, rules out the compared-away member. For enum.Flag subclasses this is unsound: a Flag value can hold any combination of members, so the declared members are not an exhaustive value set.

The reported repro:

class Programs(Flag):
    NONE = 0
    P1 = auto()
    P2 = auto()
    P3 = auto()
    ALL = P1 | P2 | P3

def f(programs: Programs) -> None:
    if programs == Programs.NONE:
        return
    if programs == Programs.P2:
        return
    if Programs.P2 in programs:  # error: Unsupported operand types for in
        ...

After the two == checks, mypy narrows programs to Literal[Programs.P1, Programs.P3, Programs.ALL] -- a type that a runtime value like Programs.P1 | Programs.P2 cannot inhabit. The subsequent Programs.P2 in programs is then rejected: typeshed defines Flag.__contains__(self, other: Self), and no member of the narrowed union accepts a P2 value, even though Programs.P2 in Programs.P1 | Programs.P2 is True at runtime.

This change skips equality/identity narrowing when either operand is a Flag value (an instance of, or a member of, an enum.Flag/enum.IntFlag subclass): the sound type for such a value is the enum class itself. Narrowing for regular enums is unchanged, since their members do form an exhaustive value set.

Checklist

• Added tests for all changed behaviour: 3 new cases in check-enum.test; they fail without the fix
• Full testcheck.py suite passes locally (8207 passed, 15 skipped, 7 xfailed); check-narrowing, check-expressions, check-literal, check-unreachable-code also pass
• mypy --config-file mypy_self_check.ini passes on the touched modules
• ruff check / ruff format --check (pinned ruff 0.14.3) pass on the touched modules

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅