Add FlushUpdates and events for players/saves debounced updates

[tudddorrr]

Debounced update completion signals

• New OnPlayerUpdated/OnPlayerUpdateFailed and OnSaveUpdated/OnSaveUpdateFailed events fire after each settled debounced batch, so callers can react to success or failure of player/save updates
• New FlushUpdates() method on both PlayersAPI and SavesAPI forces any pending trailing debounced call to execute immediately and returns an awaitable Task<bool>, enabling deterministic flush-on-quit scenarios
• The base DebouncedAPI tracks the current executing Task per operation and surfaces completion/failure through internal events, replacing the previous fire-and-forget ContinueWith pattern that only logged errors

Saves API correctness

• ExecuteDebouncedOperation for saves now returns the updated GameSave from the server and throws if no save is currently loaded (instead of silently skipping), ensuring callers always receive the canonical server state