fix(drm): key PIFF-first tracks by track ID and trust shaka's exit code

mp4decrypt copied a track whose first sinf has no tenc through encrypted; pass its key by track ID and retry with shaka on a passthrough. Shaka fails only on a non-zero exit now.

tests/test_segment_decrypt.py

@@ -23,26 +23,10 @@ class FakeDRM:
     def __init__(self) -> None:
         self.content_keys = {KID: KEY}
 
-    def mp4decrypt_key_args(self) -> list[str]:
+    def mp4decrypt_key_args(self, track_ids: object = ()) -> list[str]:
         return ["--key", f"{KID.hex}:{KEY}", "--key", f"{ZERO_KID}:{KEY}"]
 
 
-@pytest.fixture
-def stub_mp4decrypt(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
-    """An mp4decrypt that copies its input to its output and records its argv."""
-    argv_log = tmp_path / "argv.jsonl"
-    script = tmp_path / "mp4decrypt"
-    script.write_text(
-        "#!/usr/bin/env python3\n"
-        "import json, shutil, sys\n"
-        f"open({str(argv_log)!r}, 'a').write(json.dumps(sys.argv[1:]) + '\\n')\n"
-        "shutil.copyfile(sys.argv[-2], sys.argv[-1])\n"
-    )
-    script.chmod(0o755)
-    monkeypatch.setattr(binaries, "Mp4decrypt", script)
-    return argv_log
-
-
 def read_argv(argv_log: Path) -> list[list[str]]:
     return [json.loads(line) for line in argv_log.read_text().splitlines()]