Replace the timer-per-playlist + WaitGroup.Go model with a single worker
goroutine processing work items from a buffered channel. This eliminates
race conditions from parallel plugin calls (rate limiting risk),
unsynchronized map/field access across goroutines, and overlapping
discovery and refresh timers. The worker owns all mutable state
(refreshTimers, discoveryTimer) exclusively, while retryInterval and
refreshTimerCount use atomics for safe test observability. Also adds
retry-on-failure for GetAvailablePlaylists (5 min delay) and makes
MockPlaylistRepo thread-safe with sync.RWMutex.
Rename GetPlaylists → GetAvailablePlaylists for clarity. Add
PlaylistGeneratorErrorNotFound so plugins can signal a playlist is
temporarily unavailable (orchestrator skips it but retries on next
discovery). Add RetryInterval on GetAvailablePlaylistsResponse for
automatic retry of transient GetPlaylist failures.
Add a test playlist generator plugin and integration tests for the
PlaylistGenerator capability. The test plugin exercises GetPlaylists and
GetPlaylist WASM functions, and the tests verify orchestration flow
including capability detection, playlist discovery/sync, deterministic
ID generation, error handling, and graceful stop.
Also initialize playlistGenerators map in the test helper to prevent nil
map panics when loading plugins with PlaylistGenerator capability.
