fix: add url_utils.py to .fernignore (#782)

Fern regeneration was deleting this custom utility module, breaking
mypy since several files import from it.

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

Author: kraenhansen

.fernignore

@@ -9,6 +9,7 @@ src/elevenlabs/play.py
 src/elevenlabs/webhooks_custom.py
 src/elevenlabs/music_custom.py
 src/elevenlabs/speech_to_text_custom.py
+src/elevenlabs/url_utils.py
 src/elevenlabs/realtime/
 
 # Ignore CI files