FIX: remove "fake" mentions from extract_mentions (#26253)

```
<a class="mention" href="/u/test1">bsam</a>
```

Is not a mention of the user sam. We expect an @ in front always.

lib/pretty_text.rb

@@ -464,11 +464,9 @@ module PrettyText
     mentions =
       cooked
         .css(".mention, .mention-group")
-        .map do |e|
+        .filter_map do |e|
           if (name = e.inner_text)
-            name = name[1..-1]
-            name = User.normalize_username(name)
-            name
+            User.normalize_username(name[1..-1]) if name[0] == "@"
           end
         end
 

spec/lib/pretty_text_spec.rb

@@ -569,13 +569,13 @@ RSpec.describe PrettyText do
           <a class="mention" href="/u/test">@test</a>,
           <a class="mention-group" href="/g/test-group">@test-group</a>,
           <a class="custom-mention" href="/custom-mention">@test-custom</a>,
+          <a class="mention" href="/u/test1">test1</a>,
           this is a test
         </p>
         HTML
 
         extracted_mentions = PrettyText.extract_mentions(Nokogiri::HTML5.fragment(cooked_html))
-        expect(extracted_mentions).to include("test", "test-group")
-        expect(extracted_mentions).not_to include("test-custom")
+        expect(extracted_mentions).to contain_exactly("test", "test-group")
 
         Plugin::Instance
           .new