fix(node): apply a pushed inbound's client changes in place instead of rebuilding it

A master pushes the whole inbound to its node whenever a client on it is
auto-disabled, renewed or reset, and the node-sync reconcile re-sends every
inbound whose payload changed (all of them after a master restart). The
node's UpdateInbound answered every push with DelInbound + AddInbound, so
Xray closed the listener and every open connection on that port dropped.
On a node inbound with ~2,700 clients that was dozens of times a day.

UpdateInbound now diffs the config Xray is running against the new one.
When only settings.clients differ (vless, vmess, trojan, hysteria, masque)
it applies per-user RemoveUser/AddUser through the runtime, and when
nothing the core sees changed it does nothing. Any other change, or any
per-user failure (a handler the core lost included), still rebuilds as
before. AmneziaWG joins MTProto and TUIC on rt.UpdateInbound, whose
in-place Ensure keeps the device and its peers; Del+Add tore it down.

A pushed removal does not consult the node's own
restartXrayOnClientDisable: it defaults to on, and the master already
restarts its nodes from its own setting (restartRemoteNodesOnDisable).
Nodes on an older release keep rebuilding until they update; the master
side needs no change.

Verified: the new service tests, TestXrayAPI_E2E_Users against a real
core (pins the missing-user vs missing-handler error texts) and make
node-e2e. make race not run: no goroutine, lock or shared state changed.

Closes #6795

internal/xray/api.go

@@ -443,6 +443,16 @@ func IsMissingHandlerErr(err error) bool {
 		strings.Contains(msg, "not enough information")
 }
 
+// IsMissingUserErr reports whether err is xray removing a user the inbound does
+// not hold; a missing handler fails the same call and is not one.
+func IsMissingUserErr(err error) bool {
+	if err == nil {
+		return false
+	}
+	msg := strings.ToLower(err.Error())
+	return strings.Contains(msg, "not found") && !strings.Contains(msg, "failed to get handler")
+}
+
 // IsExistingTagErr reports whether err is xray's response to adding a handler
 // whose tag is already taken by a running handler.
 func IsExistingTagErr(err error) bool {