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_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)