v9.0.1

What's Changed

• artwork_url: null from the server (artless tracks, stopped playback) was silently retained by the ?? merge operator in HandleServerState. The SDK was treating explicit null and absent identically — both fell through to the existing value.
• Extended the Optional pattern from progress to all ServerMetadata fields: title, artist, album_artist, album, artwork_url, year, track, timestamp. Absent = keep existing, explicit null = clear. This matches aiosendspin's UndefinedField on every field and fixes cleared_update() on stop.
• Registered Optional<string?>, Optional<int?>, Optional<long?> in the AOT factory. Updated the "unregistered type" sentinel tests to use Optional now that string is registered.
• test: make FakeSendspinConnection connection-state-aware by @chrisuthe in #51
• Sync dev to main: connection-state test infrastructure by @chrisuthe in #52

Full Changelog: v9.0.0...v9.0.1