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>

tests/conftest.py

@@ -0,0 +1,42 @@
+"""Shared fixtures: yt-dlp is always replaced by fakes (no network)."""
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture
+def fake_yt_dlp():
+    """Factory for a subprocess.run stand-in.
+
+    It answers the info request with `info` and writes `vtt` where yt-dlp would
+    write the subtitle file. `run.calls` records every command.
+    """
+    def make(info, vtt="WEBVTT\n", write=True, fail_download=False):
+        calls = []
+
+        def run(command, capture_output, text):
+            calls.append(command)
+            if "--dump-single-json" in command:
+                return subprocess.CompletedProcess(command, 0, json.dumps(info), "")
+            if fail_download:
+                return subprocess.CompletedProcess(command, 1, "", "HTTP Error 429")
+            track = command[command.index("--sub-langs") + 1]
+            template = command[command.index("-o") + 1]
+            if write:
+                Path(template.replace("%(ext)s", f"{track}.vtt")).write_text(
+                    vtt, encoding="utf-8"
+                )
+            return subprocess.CompletedProcess(command, 0, "", "")
+
+        run.calls = calls
+        return run
+
+    return make
+
+
+@pytest.fixture
+def yt_dlp_installed(monkeypatch):
+    monkeypatch.setattr(shutil, "which", lambda name: f"/usr/bin/{name}")