Fix Enum.Equals folding for nullable boxed arguments

[Copilot]

Boxing a nullable enum can yield null while retaining an exact enum type. Enum.Equals folding currently dereferences that argument, throwing instead of returning false.

• JIT: Require a known non-null argument before comparing boxed payloads; otherwise retain the call. Preserve null-receiver exception behavior.
• Coverage: Add nullable-boxing cases for null, equal, and unequal arguments.

• Fixes JIT (bug): Enum.Equals folding throws NullReferenceException instead of returning false for a null argument #133817

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[EgorBo]

@copilot fix ci failures

[Copilot]

@copilot fix ci failures

Fixed in e2f6aba. Enum-valued [InlineData] broke assembly aliases in the merged runner; all four cases now run from TestEntryPoint. Reproduced the CI compile failure, then rebuilt JIT_others successfully and passed enum coverage in four JIT modes. The other failures in that CI run are known issues #133798 and #133645.