Remove prop rejection events, keep them inline with update results

[tudddorrr]

Result types replacing event-based error handling

• ChannelsAPI.Create and Update now return ChannelUpsertResult instead of Channel directly, wrapping success/failure status, the channel, and any rejected props.
• FeedbackAPI.Send now returns FeedbackSendResult instead of void, including success status and rejected props.
• LeaderboardsAPI.AddEntry now returns AddEntryResult instead of a tuple (LeaderboardEntry, bool), adding success status and rejected props.
• PlayersAPI no longer fires OnPropsRejected separately; rejected props are returned directly via SetProp and the existing OnPlayerUpdated event.

Removal of RejectedProp event emissions

• RejectedProp.TryEmit was removed entirely, as the pattern of emitting rejected props via events is no longer used.
• OnChannelPropsRejected and OnPropsRejected events were removed from ChannelsAPI, FeedbackAPI, LeaderboardsAPI, and PlayersAPI.

Sample updates for new API shapes

• All sample/demo scripts were updated to work with the new result types, checking Success/Entry properties instead of relying on events or tuples.