agent: ignore websocket statuses 1000, 1001 and 1005 correctly (#19172)

These are "close" messages and not actual errors.

.changelog/19172.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+agent: Correct websocket status code handling
+```

command/agent/alloc_endpoint.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"slices"
 	"strconv"
 	"strings"
 
@@ -671,6 +672,15 @@ func isClosedError(err error) bool {
 		return false
 	}
 
+	// check if the websocket "error" is one of the benign "close" status codes
+	if codedErr, ok := err.(HTTPCodedError); ok {
+		return slices.ContainsFunc([]string{
+			"close 1000", // CLOSE_NORMAL
+			"close 1001", // CLOSE_GOING_AWAY
+			"close 1005", // CLOSED_NO_STATUS
+		}, func(s string) bool { return strings.Contains(codedErr.Error(), s) })
+	}
+
 	return err == io.EOF ||
 		err == io.ErrClosedPipe ||
 		strings.Contains(err.Error(), "closed") ||