refactor: Spur-Fehlermeldung und yt-dlp-Fake gemeinsam nutzen

- yt_untertitel.no_track_message() für yt_untertitel.py und extract_text.py
- tests/conftest.py: Fixtures fake_yt_dlp und yt_dlp_installed

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

extract_text.py

@@ -8,7 +8,7 @@ import tempfile
 
 from vtt2txt import vtt_to_lines
 from yt_untertitel import (
-    YT_DLP, available_tracks, choose_track, download, fetch_info, original_language,
+    YT_DLP, choose_track, download, fetch_info, no_track_message, original_language,
 )
 
 
@@ -43,14 +43,7 @@ def main():
     lang = args.lang or original_language(info)
     chosen = choose_track(info, lang) if lang else None
     if chosen is None:
-        manual, original = available_tracks(info)
-        wanted = f"„{lang}“" if lang else "der Originalsprache"
-        raise SystemExit(
-            f"Keine Untertitel in {wanted} (weder manuell noch automatisch in "
-            f"Originalsprache).\n"
-            f"Manuell: {', '.join(manual) or '–'}\n"
-            f"Automatisch (Originalsprache): {', '.join(original) or '–'}"
-        )
+        raise SystemExit(no_track_message(info, lang))
     track, is_auto = chosen
     kind = "automatische" if is_auto else "manuelle"
     print(f"Lade {kind} Untertitel „{track}“ …", flush=True)