v3: remove deprecated automatic REST fallback in RealtimeChannel.send()

[spydon]

Summary

When a broadcast is sent through RealtimeChannel.send() while the channel cannot push over the socket (canPush == false), send() silently falls back to posting over REST to the legacy /api/broadcast endpoint. It logs a warning:

send() is automatically falling back to REST API. This behavior will be deprecated in the future. Please use httpSend() explicitly for REST delivery.

Now that httpSend() exists as the explicit REST delivery path, this implicit fallback should be removed.

Proposed change (v3)

• Remove the automatic REST fallback branch from RealtimeChannel.send().
• Callers that want REST delivery use httpSend() explicitly.

This is a behavioral breaking change, so it is scheduled for v3.

Context

Part of the SDK parity effort. Tracked under the v3 umbrella (#1278).