fix(auth): prevent mask_secret panic on multi-byte UTF-8 secrets (#591)

Use char-based indexing instead of byte-offset slicing in mask_secret()
to prevent panics when secrets contain multi-byte UTF-8 characters
(accented letters, emoji, CJK, etc.).

Co-authored-by: jpoehnelt-bot <jpoehnelt-bot@users.noreply.github.com>

.changeset/fix-mask-secret-utf8-panic.md

@@ -0,0 +1,5 @@
+---
+"@googleworkspace/cli": patch
+---
+
+Fix `mask_secret` panic on multi-byte UTF-8 secrets by using char-based indexing instead of byte-offset slicing

src/auth_commands.rs

@@ -22,17 +22,19 @@ use crate::error::GwsError;
 
 /// Mask a secret string by showing only the first 4 and last 4 characters.
 /// Strings with 8 or fewer characters are fully replaced with "***".
+///
+/// Uses char-based indexing (not byte offsets) so multi-byte UTF-8 secrets
+/// never cause a panic.
 fn mask_secret(s: &str) -> String {
     const MASK_PREFIX_LEN: usize = 4;
     const MASK_SUFFIX_LEN: usize = 4;
     const MIN_LEN_FOR_PARTIAL_MASK: usize = MASK_PREFIX_LEN + MASK_SUFFIX_LEN;
 
-    if s.len() > MIN_LEN_FOR_PARTIAL_MASK {
-        format!(
-            "{}...{}",
-            &s[..MASK_PREFIX_LEN],
-            &s[s.len() - MASK_SUFFIX_LEN..]
-        )
+    let char_count = s.chars().count();
+    if char_count > MIN_LEN_FOR_PARTIAL_MASK {
+        let prefix: String = s.chars().take(MASK_PREFIX_LEN).collect();
+        let suffix: String = s.chars().skip(char_count - MASK_SUFFIX_LEN).collect();
+        format!("{prefix}...{suffix}")
     } else {
         "***".to_string()
     }
@@ -2124,6 +2126,16 @@ mod tests {
         assert_eq!(mask_secret("123456789"), "1234...6789");
     }
 
+    #[test]
+    fn mask_secret_multibyte_utf8() {
+        // Multi-byte chars must not panic (previously used byte slicing)
+        assert_eq!(mask_secret("áéíóúñüÁÉÍÓÚ"), "áéíó...ÉÍÓÚ");
+        // Short multi-byte — should fully mask
+        assert_eq!(mask_secret("café"), "***");
+        // Exactly at boundary with multi-byte (9 Greek chars)
+        assert_eq!(mask_secret("αβγδεζηθι"), "αβγδ...ζηθι");
+    }
+
     #[test]
     fn find_unmatched_services_identifies_missing() {
         let scopes = vec![