[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/grpc.go

@@ -282,7 +282,7 @@ func (c *GrpcClient) IsHealthy() bool {
 }
 
 // WaitStreamConnected waits until the client is connected to the Signal stream
-func (c *GrpcClient) WaitStreamConnected() {
+func (c *GrpcClient) WaitStreamConnected(ctx context.Context) {
 
 	if c.status == StreamConnected {
 		return
@@ -290,6 +290,7 @@ func (c *GrpcClient) WaitStreamConnected() {
 
 	ch := c.getStreamStatusChan()
 	select {
+	case <-ctx.Done():
 	case <-c.ctx.Done():
 	case <-ch:
 	}