fix: Proxy closing listener with 0 connections left (#4389)

* fix: Proxy closing listener with 0 connections left

A bug was introduced in `0.15.0` where sessions with max connection counts were closed when the connections left hit 0. This issue was highlighted by a user: https://github.com/hashicorp/boundary/issues/4364. What was happening was when we have zero connections left the proxy code was closing the listener which closed its connections

To fix this:
- `boundary connect  -target-id=xx -host-id=xx` command with no connections left will now wait for all connections to be closed and before exiting the session just like it did in `0.14.3`
- As soon as there are no more connections left, the listener will close to stop accepting new connections
- Fix issue where the websocket connection was throwing closing errors during the session teardown.

api/proxy/proxy.go

@@ -190,7 +190,6 @@ func (p *ClientProxy) Start() (retErr error) {
 	listenerCloseFunc := func() {
 		p.listenerCloseOnce.Do(func() {
 			// Forces the for loop to exit instead of spinning on errors
-			p.cancel()
 			p.connectionsLeft.Store(0)
 			if err := p.listener.Load().(net.Listener).Close(); err != nil && err != net.ErrClosed {
 				retErr = errors.Join(retErr, fmt.Errorf("error closing proxy listener: %w", err))
@@ -228,6 +227,7 @@ func (p *ClientProxy) Start() (retErr error) {
 					// connection that comes our way, so cancel the proxy
 					fin <- fmt.Errorf("error from accept: %w", err)
 					listenerCloseFunc()
+					p.cancel()
 					return
 				}
 			}
@@ -241,6 +241,7 @@ func (p *ClientProxy) Start() (retErr error) {
 					// No reason to think we can successfully handle the next
 					// connection that comes our way, so cancel the proxy
 					listenerCloseFunc()
+					p.cancel()
 					return
 				}
 				if err := p.runTcpProxyV1(wsConn, listeningConn); err != nil {
@@ -248,6 +249,7 @@ func (p *ClientProxy) Start() (retErr error) {
 					// No reason to think we can successfully handle the next
 					// connection that comes our way, so cancel the proxy
 					listenerCloseFunc()
+					p.cancel()
 					return
 				}
 			}()
@@ -290,6 +292,7 @@ func (p *ClientProxy) Start() (retErr error) {
 	}()
 
 	p.connWg.Wait()
+	defer p.cancel()
 
 	{
 		// the go funcs are done, so we can safely close the chan and range over any errors