[client] let engine context unblock WaitStreamConnected

WaitStreamConnected only watched the signal client's own context, which
derives from the parent engineCtx rather than the engine's run context.
A Start blocked here (signal stream not yet up) could therefore not be
released by Engine.Stop, since Stop only cancels the engine's run
context.

Pass a context into WaitStreamConnected and select on it too, and have
the engine pass e.ctx, so Stop cancelling e.ctx unblocks a parked Start.
Update the Client interface, the mock, and callers accordingly.

shared/signal/client/client.go

@@ -33,7 +33,7 @@ type Client interface {
 	Receive(ctx context.Context, msgHandler func(msg *proto.Message) error) error
 	Ready() bool
 	IsHealthy() bool
-	WaitStreamConnected()
+	WaitStreamConnected(context.Context)
 	SendToStream(msg *proto.EncryptedMessage) error
 	Send(msg *proto.Message) error
 	SetOnReconnectedListener(func())