feat(dl): gate s_lang/a_lang miss behind --best-available

Missing requested subtitle and audio languages now warn and continue when --best-available is set instead of hard-exiting. Without the flag, missing languages still produce an error and exit, matching the prior strict behavior. Audio missing-lang detection is now symmetric with subtitles.

- add find_missing_langs helper in core/utilities for reuse between s_lang and a_lang paths (skips all/best/orig sentinels)
- refactor dl.py s_lang/a_lang checks to share the helper
- add tests/lang_selection covering match primitives, helper output, and tricky langcodes corners (zh-Hans/zh-Hant/zh-CN/zh-TW/zh-HK, cmn/yue, fil/tl/tgl)
- clean up unused-var ruff F841 in tests/remote/unit/

tests/remote/unit/test_compression.py

@@ -24,7 +24,6 @@ def _run(coro):
 
 
 def test_skips_when_client_does_not_accept_gzip() -> None:
-    payload = b"x" * 4096
     body_json = json.dumps({"data": "x" * 4096}).encode()
 
     async def handler(req):  # noqa: ARG001

tests/remote/unit/test_errors.py

@@ -6,13 +6,8 @@ import json
 
 import pytest
 
-from unshackle.core.api.errors import (
-    APIError,
-    APIErrorCode,
-    build_error_response,
-    categorize_exception,
-    handle_api_exception,
-)
+from unshackle.core.api.errors import (APIError, APIErrorCode, build_error_response, categorize_exception,
+                                       handle_api_exception)
 
 pytestmark = pytest.mark.unit
 

tests/remote/unit/test_handlers_serialize.py

@@ -5,16 +5,9 @@ from __future__ import annotations
 import pytest
 from langcodes import Language
 
-from unshackle.core.api.handlers import (
-    sanitize_log,
-    serialize_audio_track,
-    serialize_drm,
-    serialize_subtitle_track,
-    serialize_title,
-    serialize_video_track,
-    validate_download_parameters,
-    validate_service,
-)
+from unshackle.core.api.handlers import (sanitize_log, serialize_audio_track, serialize_drm, serialize_subtitle_track,
+                                         serialize_title, serialize_video_track, validate_download_parameters,
+                                         validate_service)
 from unshackle.core.titles.episode import Episode
 from unshackle.core.titles.movie import Movie
 from unshackle.core.tracks import Audio, Subtitle, Video

tests/remote/unit/test_remote_service_helpers.py

@@ -6,16 +6,8 @@ from enum import Enum
 
 import pytest
 
-from unshackle.core.remote_service import (
-    _build_title,
-    _build_tracks,
-    _deserialize_audio,
-    _deserialize_subtitle,
-    _deserialize_video,
-    _enum_get,
-    _match_track,
-    _reconstruct_drm,
-)
+from unshackle.core.remote_service import (_build_title, _build_tracks, _deserialize_audio, _deserialize_subtitle,
+                                           _deserialize_video, _enum_get, _match_track, _reconstruct_drm)
 from unshackle.core.titles.episode import Episode
 from unshackle.core.titles.movie import Movie
 from unshackle.core.tracks import Audio, Subtitle, Video

tests/remote/unit/test_session_store.py

@@ -105,7 +105,7 @@ async def test_get_session_store_returns_singleton() -> None:
 async def test_max_sessions_evicts_oldest(store: SessionStore, monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setattr(type(store), "_max_sessions", property(lambda _: 2))
 
-    a = await store.create("A", _FakeService(), session_id="a")
+    await store.create("A", _FakeService(), session_id="a")
     await asyncio.sleep(0.01)
     b = await store.create("B", _FakeService(), session_id="b")
     await asyncio.sleep(0.01)