FIX: only use mention styling for valid mentions in chat (#25523)

Chat should follow the same convention as topics, where invalid mentions 
are not styled the same as valid mentions. This PR makes chat use such styling.

I'm following the same pattern that we use for invalid mentions in posts – 
9bd6523581/app/assets/stylesheets/common/base/topic-post.scss (L1285-L1288)

This way it'll be easier to dry it up if we decide to do that at some point.

plugins/chat/assets/stylesheets/common/chat-message.scss

@@ -70,7 +70,7 @@
       width: 100%;
     }
 
-    .mention {
+    a.mention {
       @include mention;
 
       &.highlighted {
@@ -78,6 +78,11 @@
       }
     }
 
+    // unlinked, invalid mention
+    span.mention {
+      color: var(--primary-high);
+    }
+
     // Automatic aspect-ratio mapping https://developer.mozilla.org/en-US/docs/Web/Media/images/aspect_ratio_mapping
     p img:not(.emoji) {
       max-width: 100%;