fix(oidc): ignore unknown language tag in userinfo unmarshal

Open system reported an issue where a generic OpenID provider might return language tags like "gb".
These tags are well-formed but unknown and Go returns an error for it.
We already ignored unknown tags is ui_locale arrays lik in AuthRequest.

This change ignores singular unknown tags, like used in the userinfo `locale` claim.

pkg/oidc/types.go

@@ -3,6 +3,7 @@ package oidc
 import (
 	"database/sql/driver"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"reflect"
 	"strings"
@@ -77,7 +78,17 @@ func (l *Locale) MarshalJSON() ([]byte, error) {
 }
 
 func (l *Locale) UnmarshalJSON(data []byte) error {
-	return json.Unmarshal(data, &l.tag)
+	err := json.Unmarshal(data, &l.tag)
+	if err == nil {
+		return nil
+	}
+
+	// catch "well-formed but unknown" errors
+	var target language.ValueError
+	if errors.As(err, &target) {
+		return nil
+	}
+	return err
 }
 
 type Locales []language.Tag