gh-150220: List valid member values in Enum's default lookup ValueError

[drain99]

• Problem

>>> from enum import Enum
>>> class Currency(Enum):
...     USD = "USD"; EUR = "EUR"; JPY = "JPY"
>>> Currency("USO")
ValueError: 'USO' is not a valid Currency

The user has to inspect cls.__members__ to discover the legal set.
Pydantic and similar validators already include the list ("Input should be 'USD', 'EUR' or 'JPY'").

• Proposal

-ValueError: 'USO' is not a valid Currency
+ValueError: 'USO' is not a valid Currency. Valid values: 'USD', 'EUR', 'JPY'

…but only when _missing_ has not been overridden. Subclasses that
override _missing_ (case-insensitive matching, aliases,
Flag/IntFlag's bitwise composition, ...) own their own error
messages.

• Behaviour matrix

Case	Listing?
Enum / IntEnum / StrEnum (default _missing_)	yes
Flag / IntFlag (override for bitwise)	no
Subclass _missing_ returning None	no (subclass owns msg)
Subclass _missing_ raising ValueError	no (own message preserved)

Existing test_enum.py suite passes unchanged.

• Backward compatibility

Additive. ValueError type and prefix unchanged; only a Valid values: ... suffix.
Code asserting on the exact bare message would need a minor change.

• Issue: Enhance Enum's default ValueError to list valid member values #150220

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[picnixz]

See issue for the closing rationale.