chore: return safe copy of string slice in 'ParseStringSliceClaim' (#17439)

Claims parsed should be safe to mutate and filter. This was likely not
causing any bugs or issues, and just doing this out of precaution

coderd/idpsync/idpsync_test.go

@@ -136,6 +136,17 @@ func TestParseStringSliceClaim(t *testing.T) {
 	}
 }
 
+func TestParseStringSliceClaimReference(t *testing.T) {
+	t.Parallel()
+
+	var val any = []string{"a", "b", "c"}
+	parsed, err := idpsync.ParseStringSliceClaim(val)
+	require.NoError(t, err)
+
+	parsed[0] = ""
+	require.Equal(t, "a", val.([]string)[0], "should not modify original value")
+}
+
 func TestIsHTTPError(t *testing.T) {
 	t.Parallel()