fix: add speech_engine_custom.py to .fernignore (#789)

Fern regeneration deletes this custom module, breaking imports in
client.py. Same pattern as url_utils.py fix in #782.

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

Author: kraenhansen

.fernignore

@@ -12,6 +12,7 @@ src/elevenlabs/speech_to_text_custom.py
 src/elevenlabs/url_utils.py
 src/elevenlabs/realtime/
 src/elevenlabs/speech_engine/
+src/elevenlabs/speech_engine_custom.py
 
 # Ignore CI files
 .github/