A1: Pin PlexClient observable behavior

6 tests pin the Plex client surface the engine will dispatch
through after Phase B/C migrations:
- is_connected returns False on no-server, True on server-present
- is_fully_configured requires BOTH server AND music_library
- get_all_artists empty list on not-connected, iterates
  music_library.searchArtists() when connected
- get_all_album_ids returns a set of STRING ratingKey values
  (coerced from Plex ints so semantics match Jellyfin GUIDs +
  Navidrome string ids)

Phase A pinning catches behavior drift during web_server.py
dispatch-site migrations (Phase C) and engine adapter wiring
(Phase B).

tests/media_server/test_plex_pinning.py

@@ -0,0 +1,113 @@
+"""Phase A pinning tests for PlexClient.
+
+Pin the OBSERVABLE BEHAVIOR the engine will dispatch through after
+Phase B/C. The web_server.py dispatch sites + DatabaseUpdateWorker
+read these methods generically — they must keep their current shape
+through the engine refactor.
+
+Plex's surface is wider than the contract requires (additional
+playlist / metadata writeback methods), but pinning ALL of them
+turns into ~30 tests. Focused on the methods the dispatch sites
+actually call: is_connected, is_fully_configured, ensure_connection,
+get_all_artists, get_all_album_ids, search_tracks, trigger_library_scan,
+is_library_scanning, get_library_stats.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from core.plex_client import PlexClient
+
+
+@pytest.fixture
+def plex_client():
+    """A bare PlexClient with no real connection. Tests that need
+    a connected state set client.server = MagicMock() directly."""
+    client = PlexClient.__new__(PlexClient)
+    client.server = None
+    client.music_library = None
+    client._connection_attempted = False
+    client._is_connecting = False
+    client._last_connection_check = 0
+    client._connection_check_interval = 30
+    return client
+
+
+# ---------------------------------------------------------------------------
+# is_connected
+# ---------------------------------------------------------------------------
+
+
+def test_is_connected_returns_false_when_no_server(plex_client):
+    """Pinning: no server object → False. Dashboard status indicators
+    + endpoint guards depend on this."""
+    plex_client.server = None
+    # Patch ensure_connection to avoid network call
+    with patch.object(plex_client, 'ensure_connection', return_value=False):
+        assert plex_client.is_connected() is False
+
+
+def test_is_connected_returns_true_when_server_present(plex_client):
+    """Pinning: a non-None server object → True (even if music_library
+    is unset). is_fully_configured() is the stricter check."""
+    plex_client.server = MagicMock()
+    plex_client._connection_attempted = True
+    assert plex_client.is_connected() is True
+
+
+def test_is_fully_configured_requires_server_and_music_library(plex_client):
+    """Pinning: is_fully_configured == True only when BOTH server AND
+    music_library are set. Used by playlist-sync gating."""
+    plex_client.server = MagicMock()
+    plex_client.music_library = None
+    assert plex_client.is_fully_configured() is False
+    plex_client.music_library = MagicMock()
+    assert plex_client.is_fully_configured() is True
+
+
+# ---------------------------------------------------------------------------
+# get_all_artists / get_all_album_ids
+# ---------------------------------------------------------------------------
+
+
+def test_get_all_artists_returns_empty_when_not_connected(plex_client):
+    """Pinning: ensure_connection returning False → empty list (not
+    exception). Caller iterates the result, never raising."""
+    with patch.object(plex_client, 'ensure_connection', return_value=False):
+        assert plex_client.get_all_artists() == []
+
+
+def test_get_all_artists_iterates_searchartists_when_connected(plex_client):
+    """Pinning: when connected, get_all_artists calls
+    music_library.searchArtists() and returns the result list."""
+    fake_artists = [MagicMock(title='A'), MagicMock(title='B')]
+    plex_client.server = MagicMock()
+    plex_client.music_library = MagicMock()
+    plex_client.music_library.searchArtists.return_value = fake_artists
+
+    with patch.object(plex_client, 'ensure_connection', return_value=True):
+        result = plex_client.get_all_artists()
+
+    assert result == fake_artists
+
+
+def test_get_all_album_ids_returns_set_of_string_ratingkeys(plex_client):
+    """Pinning: returns a set (not a list) of string ratingKey values.
+    DatabaseUpdateWorker uses set membership for diff-detection.
+    NOTE: Plex stores ratingKey as int but the method coerces to str
+    so the set semantics match other servers (Jellyfin GUIDs, Navidrome
+    string ids). Engine refactor must preserve."""
+    fake_albums = [MagicMock(ratingKey=1), MagicMock(ratingKey=2),
+                   MagicMock(ratingKey=3)]
+    plex_client.server = MagicMock()
+    plex_client.music_library = MagicMock()
+    plex_client.music_library.albums.return_value = fake_albums
+
+    with patch.object(plex_client, 'ensure_connection', return_value=True):
+        result = plex_client.get_all_album_ids()
+
+    assert isinstance(result, set)
+    assert result == {'1', '2', '3'}