fix: yt-dlp im temporären Verzeichnis ausführen (Streudatei „3“)

yt-dlp ruft zur Prüfung „ffmpeg -bsfs“ auf; das System-ffmpeg 6.1.1
(Ubuntu) legt dabei eine leere Datei „3“ im Arbeitsverzeichnis an.
yt-dlp läuft jetzt in einem temporären Verzeichnis mit absolutem
Zielpfad, die Datei verschwindet mit dem Verzeichnis.

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

tests/test_yt_untertitel.py

@@ -1,6 +1,7 @@
 """Tests for yt_untertitel.py; yt-dlp is always replaced by fakes (no network)."""
 import subprocess
 import sys
+from pathlib import Path
 
 import pytest
 
@@ -67,6 +68,19 @@ def test_main_downloads_auto_original(fake_yt_dlp, tmp_path, monkeypatch, capsys
     assert "vtt2txt.py -i" in out
 
 
+def test_yt_dlp_runs_in_temporary_directory(fake_yt_dlp, tmp_path, monkeypatch):
+    # ffmpeg -bsfs, called by yt-dlp, leaves a file "3" in its working directory
+    run = fake_yt_dlp(AUTO_ONLY)
+    monkeypatch.setattr(yt.subprocess, "run", run)
+    monkeypatch.chdir(tmp_path)
+    run_main(monkeypatch, "-u", URL, "-n", "Rede")
+
+    assert all(cwd is not None and Path(cwd) != tmp_path for cwd in run.cwds)
+    template = run.calls[1][run.calls[1].index("-o") + 1]
+    assert template == f"{tmp_path / 'Rede'}.%(ext)s"
+    assert (tmp_path / "Rede.de-orig.vtt").is_file()
+
+
 def test_main_prefers_manual_and_defaults_to_video_id(fake_yt_dlp, tmp_path, monkeypatch):
     info = dict(AUTO_ONLY, subtitles={"de": []})
     run = fake_yt_dlp(info)
@@ -84,7 +98,7 @@ def test_main_no_matching_track(fake_yt_dlp, tmp_path, monkeypatch):
 
 
 def test_main_info_error(tmp_path, monkeypatch):
-    def run(command, capture_output, text):
+    def run(command, capture_output, text, cwd=None):
         return subprocess.CompletedProcess(command, 1, "", "ERROR: Video unavailable")
 
     monkeypatch.setattr(yt.subprocess, "run", run)