workload/schemachanger: ignore unique violation errors for CREATE INDEX

Previously, the schema changer workload added logic to ignore potential
unique violation errors during CREATE UNIQUE INDEX. This logic was for
the statement phase, which is not when this error would be noticed, so
it did not work correctly. To address this, this patch adds the logic to
make unique violations error as  potential commit error for CREATE
INDEX.

Fixes: #129366

Release note: None

pkg/workload/schemachange/operation_generator.go

@@ -1118,7 +1118,7 @@ func (og *operationGenerator) createIndex(ctx context.Context, tx pgx.Tx) (*opSt
 	// When an index exists, but `IF NOT EXISTS` is used, then
 	// the index will not be created and the op will complete without errors.
 	if !(indexExists && def.IfNotExists) {
-		stmt.potentialExecErrors.addAll(codesWithConditions{
+		og.potentialCommitErrors.addAll(codesWithConditions{
 			// If there is data in the table such that a unique index cannot be created,
 			// a pgcode.UniqueViolation will occur and will be wrapped in a
 			// pgcode.TransactionCommittedWithSchemaChangeFailure. The schemachange worker