A2: Pin JellyfinClient observable behavior

5 tests pin Jellyfin client surface. is_connected requires ALL
four of base_url + api_key + user_id + music_library_id (stricter
than Plex's is_connected). get_all_album_ids returns a set of
string GUID ids matching the cross-server uniform set semantics.

tests/media_server/test_jellyfin_pinning.py

@@ -0,0 +1,104 @@
+"""Phase A pinning tests for JellyfinClient.
+
+Pin the OBSERVABLE BEHAVIOR the engine will dispatch through after
+Phase B/C. Jellyfin's connection-readiness check is stricter than
+Plex (requires base_url + api_key + user_id + music_library_id all
+present), and get_all_artists has a side effect of pre-populating
+caches.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from core.jellyfin_client import JellyfinClient
+
+
+@pytest.fixture
+def jellyfin_client():
+    """A bare JellyfinClient with no real connection."""
+    client = JellyfinClient.__new__(JellyfinClient)
+    client.base_url = None
+    client.api_key = None
+    client.user_id = None
+    client.music_library_id = None
+    client._connection_attempted = False
+    client._is_connecting = False
+    # Initialize the various caches the real __init__ sets up
+    client._album_cache = {}
+    client._track_cache = {}
+    client._artist_cache = {}
+    client._cache_populated = False
+    client._metadata_only_mode = False
+    return client
+
+
+# ---------------------------------------------------------------------------
+# is_connected
+# ---------------------------------------------------------------------------
+
+
+def test_is_connected_returns_false_when_no_credentials(jellyfin_client):
+    """Pinning: any of base_url / api_key / user_id / music_library_id
+    being None → False. All four must be set."""
+    with patch.object(jellyfin_client, 'ensure_connection', return_value=False):
+        assert jellyfin_client.is_connected() is False
+
+
+def test_is_connected_returns_true_only_when_all_four_set(jellyfin_client):
+    """Pinning: requires base_url AND api_key AND user_id AND
+    music_library_id. Setting only some → still False."""
+    jellyfin_client._connection_attempted = True
+
+    # Only some — still False
+    jellyfin_client.base_url = 'http://j'
+    jellyfin_client.api_key = 'k'
+    jellyfin_client.user_id = 'u'
+    jellyfin_client.music_library_id = None
+    assert jellyfin_client.is_connected() is False
+
+    # All four — True
+    jellyfin_client.music_library_id = 'lib'
+    assert jellyfin_client.is_connected() is True
+
+
+# ---------------------------------------------------------------------------
+# get_all_artists / get_all_album_ids
+# ---------------------------------------------------------------------------
+
+
+def test_get_all_artists_returns_empty_when_not_connected(jellyfin_client):
+    with patch.object(jellyfin_client, 'ensure_connection', return_value=False):
+        assert jellyfin_client.get_all_artists() == []
+
+
+def test_get_all_artists_returns_empty_when_no_music_library_id(jellyfin_client):
+    """Pinning: even with ensure_connection True, no music_library_id
+    selected → empty (don't crash)."""
+    jellyfin_client.music_library_id = None
+    with patch.object(jellyfin_client, 'ensure_connection', return_value=True):
+        assert jellyfin_client.get_all_artists() == []
+
+
+def test_get_all_album_ids_returns_set(jellyfin_client):
+    """Pinning: returns a set of string Jellyfin GUID ids. Same shape
+    semantic as Plex (set of strings) — engine extraction depends on
+    uniform set type."""
+    jellyfin_client._connection_attempted = True
+    jellyfin_client.base_url = 'http://j'
+    jellyfin_client.api_key = 'k'
+    jellyfin_client.user_id = 'u'
+    jellyfin_client.music_library_id = 'lib'
+
+    fake_response_pages = [
+        {'Items': [{'Id': 'guid-1'}, {'Id': 'guid-2'}], 'TotalRecordCount': 2},
+    ]
+
+    with patch.object(jellyfin_client, 'ensure_connection', return_value=True), \
+         patch.object(jellyfin_client, '_make_request', side_effect=fake_response_pages):
+        result = jellyfin_client.get_all_album_ids()
+
+    assert isinstance(result, set)
+    assert result == {'guid-1', 'guid-2'}