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_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