Properly normalize completion labels with multi-byte characters (#25150)

Closes https://github.com/zed-industries/zed/issues/25142

Release Notes:

- Fixed panics on completion with multi-byte characters input

crates/project/src/lsp_store.rs

@@ -8944,7 +8944,7 @@ fn ensure_uniform_list_compatible_label(label: &mut CodeLabel) {
             }
             _ => {
                 new_text.push(c);
-                new_idx += 1;
+                new_idx += c.len_utf8();
                 last_char_was_space = false;
             }
         }
@@ -9015,6 +9015,11 @@ fn ensure_uniform_list_compatible_label(label: &mut CodeLabel) {
 }
 
 #[cfg(test)]
+mod tests {
+    use language::HighlightId;
+
+    use super::*;
+
     #[test]
     fn test_glob_literal_prefix() {
         assert_eq!(glob_literal_prefix(Path::new("**/*.js")), Path::new(""));
@@ -9048,3 +9053,22 @@ fn test_glob_literal_prefix() {
             );
         }
     }
+
+    #[test]
+    fn test_multi_len_chars_normalization() {
+        let mut label = CodeLabel {
+            text: "myElˇ (parameter) myElˇ: {\n    foo: string;\n}".to_string(),
+            runs: vec![(0..6, HighlightId(1))],
+            filter_range: 0..6,
+        };
+        ensure_uniform_list_compatible_label(&mut label);
+        assert_eq!(
+            label,
+            CodeLabel {
+                text: "myElˇ (parameter) myElˇ: { foo: string; }".to_string(),
+                runs: vec![(0..6, HighlightId(1))],
+                filter_range: 0..6,
+            }
+        );
+    }
+}