ML-193: add secondary parser, search, and API coverage tests

Add ~80 focused tests across 10 files covering:

- Record set search by name, description, contained record title, artist name
- Navigation events for records, artists, record sets, links, and view-all actions
- MusicBrainz.ReleaseGroup parser (artist credits, included RGs, release IDs, types)
- MusicBrainz.ReleaseSearchResult edge cases (missing RG, unknown formats)
- LastFm.Session XML edge cases (non-subscriber, missing nodes)
- MusicLibrary.HttpError default status-kind mapping
- Discogs.API.ErrorResponse fallback message and retry-delay behavior
- ArtistRefreshWikipediaData {:cancel, :no_english_wikipedia} worker path
- StatsComponents grouped on-this-day records and anniversary labels

test/last_fm/session_test.exs

@@ -2,4 +2,98 @@ defmodule LastFm.SessionTest do
   use ExUnit.Case, async: true
 
   doctest LastFm.Session
+
+  alias LastFm.Session
+
+  describe "parse/1" do
+    test "non-subscriber XML" do
+      xml = """
+      <?xml version="1.0" encoding="UTF-8"?>
+      <lfm status="ok">
+        <session>
+          <name>freeuser</name>
+          <key>free-key-123</key>
+          <subscriber>0</subscriber>
+        </session>
+      </lfm>
+      """
+
+      assert Session.parse(xml) == %Session{
+               name: "freeuser",
+               key: "free-key-123",
+               pro: false
+             }
+    end
+
+    test "missing subscriber node defaults pro to false" do
+      xml = """
+      <?xml version="1.0" encoding="UTF-8"?>
+      <lfm status="ok">
+        <session>
+          <name>basicuser</name>
+          <key>basic-key-456</key>
+        </session>
+      </lfm>
+      """
+
+      assert Session.parse(xml) == %Session{
+               name: "basicuser",
+               key: "basic-key-456",
+               pro: false
+             }
+    end
+
+    test "missing name node returns nil name" do
+      xml = """
+      <?xml version="1.0" encoding="UTF-8"?>
+      <lfm status="ok">
+        <session>
+          <key>some-key</key>
+          <subscriber>1</subscriber>
+        </session>
+      </lfm>
+      """
+
+      assert Session.parse(xml) == %Session{
+               name: nil,
+               key: "some-key",
+               pro: true
+             }
+    end
+
+    test "missing key node returns nil key" do
+      xml = """
+      <?xml version="1.0" encoding="UTF-8"?>
+      <lfm status="ok">
+        <session>
+          <name>mysteryuser</name>
+          <subscriber>0</subscriber>
+        </session>
+      </lfm>
+      """
+
+      assert Session.parse(xml) == %Session{
+               name: "mysteryuser",
+               key: nil,
+               pro: false
+             }
+    end
+
+    test "missing both name and key nodes returns nil values" do
+      xml = """
+      <?xml version="1.0" encoding="UTF-8"?>
+      <lfm status="ok">
+        <session>
+          <subscriber>1</subscriber>
+        </session>
+      </lfm>
+      """
+
+      assert Session.parse(xml) == %Session{
+               name: nil,
+               key: nil,
+               pro: true
+             }
+    end
+  end
 end