test(youtube): add unit tests for the YouTubeChannel readiness check

YouTube's readiness is layered, so 13 tests stub probe_command /
shutil.which to exercise every branch offline:

- can_handle across youtube.com / youtu.be hosts (www/m subdomains,
  case-insensitive) + rejection of unrelated hosts.
- _has_js_runtime_config: missing file -> False, "--js-runtimes" present
  -> True, absent -> False, and OSError on read -> False.
- check() probe states: missing -> off, broken -> error (hint surfaced),
  timeout/unrunnable -> error; active_backend cleared in all three.
- check() JS-runtime gating: no deno/node -> warn, Node-only with config
  missing the flag -> warn, deno present -> ok; once the probe is ok the
  backend is attributed (yt-dlp) regardless of the JS/transcription nuance.
- check() transcription readiness: a configured whisper provider with
  ffmpeg present reports it, and a missing ffmpeg is flagged.

Follow-up to #331 — extends dedicated channel coverage after rss (#360),
github (#361), web (#363), reddit (#364), xueqiu (#365) and v2ex (#366).

tests/test_youtube_channel.py

@@ -0,0 +1,147 @@
+# -*- coding: utf-8 -*-
+"""Dedicated tests for the ``youtube`` channel.
+
+YouTube's readiness is layered: yt-dlp must probe alive (missing / broken /
+unrunnable are distinct), a JS runtime is mandatory (deno works out of the
+box, Node needs an explicit ``--js-runtimes`` config), and transcription
+readiness (whisper provider + ffmpeg) is surfaced for ``doctor`` without
+gating the backend. These tests stub ``probe_command`` / ``shutil.which``
+so every branch runs offline. Follow-up to #331 — extends dedicated
+channel coverage after rss (#360), github (#361), web (#363),
+reddit (#364), xueqiu (#365) and v2ex (#366).
+"""
+
+from unittest.mock import Mock, patch
+
+from agent_reach.probe import ProbeResult
+from agent_reach.channels import youtube as yt
+from agent_reach.channels.youtube import YouTubeChannel, _has_js_runtime_config
+
+
+def _which(*present):
+    """shutil.which side effect: returns a path for the named tools present."""
+    return lambda name: f"/usr/bin/{name}" if name in present else None
+
+
+# --- can_handle ---
+
+def test_can_handle_matches_youtube_hosts():
+    ch = YouTubeChannel()
+    for url in [
+        "https://www.youtube.com/watch?v=abc",
+        "https://youtu.be/abc",
+        "https://m.youtube.com/watch?v=abc",
+        "https://YOUTUBE.COM/shorts/x",
+    ]:
+        assert ch.can_handle(url) is True, url
+    for url in ["https://example.com", "https://vimeo.com/123", ""]:
+        assert ch.can_handle(url) is False, url
+
+
+# --- _has_js_runtime_config ---
+
+def test_has_js_runtime_config_missing_file_is_false(tmp_path):
+    assert _has_js_runtime_config(tmp_path / "nope.conf") is False
+
+
+def test_has_js_runtime_config_true_when_flag_present(tmp_path):
+    cfg = tmp_path / "config"
+    cfg.write_text("--js-runtimes deno\n--no-mtime\n", encoding="utf-8")
+    assert _has_js_runtime_config(cfg) is True
+
+
+def test_has_js_runtime_config_false_when_flag_absent(tmp_path):
+    cfg = tmp_path / "config"
+    cfg.write_text("--no-mtime\n", encoding="utf-8")
+    assert _has_js_runtime_config(cfg) is False
+
+
+def test_has_js_runtime_config_swallows_oserror():
+    bad = Mock()
+    bad.exists.return_value = True
+    with patch.object(yt, "read_utf8_text", side_effect=OSError("denied")):
+        assert _has_js_runtime_config(bad) is False
+
+
+# --- check(): yt-dlp probe states ---
+
+def test_check_off_when_ytdlp_missing():
+    ch = YouTubeChannel()
+    with patch.object(yt, "probe_command", return_value=ProbeResult("missing")):
+        status, message = ch.check()
+    assert status == "off"
+    assert ch.active_backend is None
+
+
+def test_check_error_when_ytdlp_broken():
+    ch = YouTubeChannel()
+    with patch.object(yt, "probe_command", return_value=ProbeResult("broken", hint="relink venv")):
+        status, message = ch.check()
+    assert status == "error"
+    assert "relink venv" in message
+    assert ch.active_backend is None
+
+
+def test_check_error_when_ytdlp_unrunnable():
+    ch = YouTubeChannel()
+    with patch.object(yt, "probe_command", return_value=ProbeResult("timeout", hint="too slow")):
+        status, message = ch.check()
+    assert status == "error"
+    assert ch.active_backend is None
+
+
+# --- check(): JS runtime gating (backend stays yt-dlp once the probe is ok) ---
+
+def test_check_warn_when_no_js_runtime_but_backend_active():
+    ch = YouTubeChannel()
+    with patch.object(yt, "probe_command", return_value=ProbeResult("ok")), \
+         patch("shutil.which", side_effect=_which()):  # no deno, no node
+        status, message = ch.check()
+    assert status == "warn"
+    assert "JS runtime" in message
+    assert ch.active_backend == "yt-dlp"  # probe was ok → backend attributed
+
+
+def test_check_warn_when_node_only_and_config_missing_flag():
+    ch = YouTubeChannel()
+    with patch.object(yt, "probe_command", return_value=ProbeResult("ok")), \
+         patch("shutil.which", side_effect=_which("node")), \
+         patch.object(yt, "_has_js_runtime_config", return_value=False):
+        status, message = ch.check()
+    assert status == "warn"
+    assert ch.active_backend == "yt-dlp"
+
+
+def test_check_ok_with_deno():
+    ch = YouTubeChannel()
+    with patch.object(yt, "probe_command", return_value=ProbeResult("ok")), \
+         patch("shutil.which", side_effect=_which("deno")):
+        status, message = ch.check()
+    assert status == "ok"
+    assert message == "可提取视频信息和字幕"
+    assert ch.active_backend == "yt-dlp"
+
+
+# --- check(): transcription readiness surfaced (deno present so JS path is ok) ---
+
+def test_check_ok_reports_transcription_when_provider_and_ffmpeg_present():
+    ch = YouTubeChannel()
+    cfg = Mock()
+    cfg.is_configured = lambda key: key == "groq_whisper"
+    with patch.object(yt, "probe_command", return_value=ProbeResult("ok")), \
+         patch("shutil.which", side_effect=_which("deno", "ffmpeg")):
+        status, message = ch.check(config=cfg)
+    assert status == "ok"
+    assert "groq" in message
+    assert "可转写音频" in message
+
+
+def test_check_ok_flags_missing_ffmpeg_for_transcription():
+    ch = YouTubeChannel()
+    cfg = Mock()
+    cfg.is_configured = lambda key: key == "openai_whisper"
+    with patch.object(yt, "probe_command", return_value=ProbeResult("ok")), \
+         patch("shutil.which", side_effect=_which("deno")):  # provider yes, ffmpeg no
+        status, message = ch.check(config=cfg)
+    assert status == "ok"
+    assert "ffmpeg" in message