Fix intermittent panic in Close from test scenarios

[peterbroadhurst]

In chain with #240

In use of this library with the background connection option, we have intermittent panics in unit tests during cleanup as so due to management of the Close function not using a mutex:

goroutine 69 [running]:
github.com/hyperledger-firefly/common/pkg/wsclient.(*wsClient).Close(0x2438b0455d00)
	/home/runner/go/pkg/mod/github.com/hyperledger-firefly/common@v1.6.6-0.20260727155622-37f809c40b02/pkg/wsclient/wsclient.go:280 +0x63
github.com/hyperledger-firefly/common/pkg/wsclient.New.func1()
	/home/runner/go/pkg/mod/github.com/hyperledger-firefly/common@v1.6.6-0.20260727155622-37f809c40b02/pkg/wsclient/wsclient.go:200 +0x11d
created by github.com/hyperledger-firefly/common/pkg/wsclient.New in goroutine 68
	/home/runner/go/pkg/mod/github.com/hyperledger-firefly/common@v1.6.6-0.20260727155622-37f809c40b02/pkg/wsclient/wsclient.go:196 +0x80a