RFR: 8373288: EnumMap spliterators should include more specific characteristics

[ExE Boss]

On Tue, 16 Dec 2025 04:42:53 GMT, Patrick Strawderman <duke at openjdk.org> wrote:

> The spliterators returned by keySet, entrySet, and values were not reporting additional characteristics, such as ORDERED, SORTED, and NONNULL; the EnumMap iterators all guarantee iteration is in the natural order of the enum key, and nulls are disallowed as keys.
> 
> Null values are allowed, so the values collection can only additionally report ORDERED.

Note that unlike `EnumSet`, `EnumMap` can be subclassed by arbitrary user code.

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PR Comment: https://git.openjdk.org/jdk/pull/28840#issuecomment-3677520831