mcp: fix goroutine leaks in unit tests (#496)

This PR fixes goroutine leaks in all unit tests.

To find the leaks I integrated https://github.com/uber-go/goleak and I
suggest to keep using it to catch any future regressions.

I used the folowing script to more easily find individual tests which
were leaking goroutines:

```
for t in $(go test -run=Nothing -list=. ./mcp | grep -v ok); do go test ./mcp -run="$t" > /dev/null && echo -n . || echo -e "\nDetected leak: $t"; done
```

mcp/server.go

@@ -778,6 +778,7 @@ func (s *Server) Run(ctx context.Context, t Transport) error {
 	select {
 	case <-ctx.Done():
 		ss.Close()
+		<-ssClosed // wait until waiting go routine above actually completes
 		s.opts.Logger.Error("server run cancelled", "error", ctx.Err())
 		return ctx.Err()
 	case err := <-ssClosed: