feat(subtitle): support 'original' sub_format to keep source format

tests/core/test_click_types.py

@@ -0,0 +1,33 @@
+"""Tests for SubtitleCodecChoice — notably the ``original`` keep-source sentinel that
+services set via the ``sub_format`` override (must not be rejected as an invalid codec)."""
+
+from __future__ import annotations
+
+import pytest
+
+from unshackle.core.tracks.subtitle import Subtitle
+from unshackle.core.utils.click_types import SubtitleCodecChoice
+
+choice = SubtitleCodecChoice(Subtitle.Codec)
+
+
+@pytest.mark.parametrize("value", ["original", "ORIGINAL", "Original"])
+def test_original_is_kept_as_sentinel(value):
+    assert choice.convert(value) == "original"
+
+
+@pytest.mark.parametrize(
+    "value,expected",
+    [
+        ("srt", Subtitle.Codec.SubRip),
+        ("ass", Subtitle.Codec.SubStationAlphav4),
+        ("vtt", Subtitle.Codec.WebVTT),
+        ("WVTT", Subtitle.Codec.fVTT),
+    ],
+)
+def test_codecs_still_map(value, expected):
+    assert choice.convert(value) == expected
+
+
+def test_empty_is_none():
+    assert choice.convert(None) is None