[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/mock.go

@@ -55,7 +55,7 @@ func (sm *MockClient) Ready() bool {
 	return sm.ReadyFunc()
 }
 
-func (sm *MockClient) WaitStreamConnected() {
+func (sm *MockClient) WaitStreamConnected(context.Context) {
 	if sm.WaitStreamConnectedFunc == nil {
 		return
 	}