Fix Enum.Equals folding for shared-generic nested enums

[Copilot]

Enum.Equals can incorrectly equate C<string>.E and C<object>.E because their canonical handles match despite distinct runtime types.

• JIT: Reject shared-instance types before folding to a payload comparison, matching the Enum.HasFlag guard.
• Regression coverage: Add parameterized cases for equal and unequal payloads across matching and differing shared/non-shared enum types.

• Fixes JIT: (bug) Enum.Equals folding returns true for different shared-generic nested enum types #134890

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[EgorBo]

PTAL @jakobbotsch @dotnet/jit-contrib no diffs

[EgorBo]

/ba-g #135031