Fix array api support in `pos_label` checking in brier score metrics

[lucyleeow]

PR # 32422 added support for brier metrics, but missed array API support when doing checking pos_label here:

scikit-learn/sklearn/metrics/_classification.py

Line 3543 in d3cd361

classes = np.unique(y_true)

np here should be xp

Also add tests for this.