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>

yt_untertitel.py

@@ -56,6 +56,18 @@ def available_tracks(info):
     return manual, original
 
 
+def no_track_message(info, lang):
+    """Explain that no usable track exists for `lang` (None: original language)."""
+    manual, original = available_tracks(info)
+    wanted = f"„{lang}“" if lang else "der Originalsprache"
+    return (
+        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 '–'}"
+    )
+
+
 def download(url, track, is_auto, target_base):
     """Download one subtitle track; yt-dlp writes <target_base>.<track>.vtt."""
     command = [
@@ -103,13 +115,7 @@ def main():
 
     chosen = choose_track(info, args.lang)
     if chosen is None:
-        manual, original = available_tracks(info)
-        raise SystemExit(
-            f"Keine Untertitel in „{args.lang}“ (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, args.lang))
     track, is_auto = chosen
     kind = "automatische" if is_auto else "manuelle"
     print(f"Lade {kind} Untertitel „{track}“ …", flush=True)