Handle 404 errors gracefully in AvatarsController (#8346)

When a user's avatar_url points to a dead link, the open-uri fetch
raises OpenURI::HTTPError. Previously this was caught by the broad
StandardError rescue, reported to Bugsnag (noise), and returned a 500.

Now we specifically rescue OpenURI::HTTPError and fall back to serving
the placeholder avatar with a short cache TTL (5 minutes), so users
still see an avatar and the error doesn't flood Bugsnag.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: iHiD

app/controllers/avatars_controller.rb

@@ -29,6 +29,11 @@ def show
     send_data data, type: content_type, disposition: 'inline'
   rescue ActiveRecord::RecordNotFound
     head :not_found
+  rescue OpenURI::HTTPError
+    url = "#{Exercism.config.website_icons_host}/placeholders/user-avatar.svg"
+    file = URI.parse(url).open
+    expires_in 5.minutes, public: true
+    send_data file.read, type: file.content_type, disposition: 'inline'
   rescue StandardError => e
     Bugsnag.notify(e)
     head :internal_server_error