Fix ImmutableHashSet.SetEquals correctness for mismatched comparers

[aw0lid]

Summary

This PR fixes a correctness issue in SetEquals introduced in my previous PR #126309.

The Count check was moved inside the Comparer equality block. This ensures that when comparers differ, we don't return a false negative and instead fall back to the safe path.

Example of the issue fixed:

// This should return true, but was returning false
var main = ImmutableHashSet.Create(StringComparer.OrdinalIgnoreCase, "a");
var other = ImmutableHashSet.Create("a", "A");
Console.WriteLine(main.SetEquals(other));

Changes

• Moved if (count != origin.Count) inside the EqualityComparer<IEqualityComparer<T>>.Default.Equals check.
• This ensures mismatched comparers safely bypass the fast-path and proceed to a proper set comparison.

Related to: #126309

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