A3: Pin NavidromeClient observable behavior

4 tests pin the Navidrome client surface. Auth shape: base_url +
username + password (no token model — salt generated per request).
get_all_album_ids paginates getAlbumList2 and returns a set of
string ids matching cross-server uniform set semantics.

tests/media_server/test_navidrome_pinning.py

@@ -0,0 +1,79 @@
+"""Phase A pinning tests for NavidromeClient.
+
+Pin the OBSERVABLE BEHAVIOR the engine will dispatch through. Auth
+shape uses base_url + username + password (no token like Plex).
+Both get_all_artists + get_all_album_ids return empty when the
+client isn't connected.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from core.navidrome_client import NavidromeClient
+
+
+@pytest.fixture
+def nav_client():
+    """A bare NavidromeClient with no real connection."""
+    client = NavidromeClient.__new__(NavidromeClient)
+    client.base_url = None
+    client.username = None
+    client.password = None
+    client._connection_attempted = False
+    client._is_connecting = False
+    client._artist_cache = {}
+    client._album_cache = {}
+    client._track_cache = {}
+    client._folder_album_ids = None
+    client._progress_callback = None
+    client.music_folder_id = None
+    return client
+
+
+def test_is_connected_returns_false_without_credentials(nav_client):
+    with patch.object(nav_client, 'ensure_connection', return_value=False):
+        assert nav_client.is_connected() is False
+
+
+def test_is_connected_returns_true_when_all_three_set(nav_client):
+    """Pinning: requires base_url AND username AND password.
+    No token model (vs Plex). Salt is generated per-request."""
+    nav_client._connection_attempted = True
+    nav_client.base_url = 'http://nav'
+    nav_client.username = 'u'
+    nav_client.password = 'p'
+    assert nav_client.is_connected() is True
+
+
+def test_get_all_artists_returns_empty_when_not_connected(nav_client):
+    with patch.object(nav_client, 'ensure_connection', return_value=False):
+        assert nav_client.get_all_artists() == []
+
+
+def test_get_all_album_ids_returns_set(nav_client):
+    """Pinning: returns a set of string Navidrome ids. Same shape
+    semantic as Plex/Jellyfin (set of strings) — engine extraction
+    depends on uniform set type."""
+    nav_client._connection_attempted = True
+    nav_client.base_url = 'http://nav'
+    nav_client.username = 'u'
+    nav_client.password = 'p'
+
+    # Navidrome uses paginated getAlbumList2 — first page returns
+    # 2 albums, second returns empty (terminates loop).
+    # _make_request unwraps the subsonic-response envelope and
+    # returns the body. get_all_album_ids reads response.albumList2.album.
+    page_responses = [
+        {'albumList2': {'album': [{'id': 'nav-1'}, {'id': 'nav-2'}]}},
+        {'albumList2': {'album': []}},
+    ]
+
+    with patch.object(nav_client, 'ensure_connection', return_value=True), \
+         patch.object(nav_client, '_make_request', side_effect=page_responses):
+        result = nav_client.get_all_album_ids()
+
+    assert isinstance(result, set)
+    assert result == {'nav-1', 'nav-2'}