Detect and surface invalid or stale gateway in Path Browser/Path History

NetBrain currently does not detect when a path's associated gateway has become invalid or stale, such as when the gateway reference was imported from a CSV file and no longer resolves correctly. When this occurs, the path calculation throws an exception and Path Browser/Path History simply displays N/A, giving no indication that an invalid gateway is the underlying cause. This enhancement would improve the path-handling logic to detect an invalid or stale gateway condition and either automatically re-resolve the gateway or clearly surface the invalid-gateway state to the user in Path Browser/Path History, rather than returning an unexplained N/A. Customers would benefit from faster root-cause identification and would avoid the manual workaround of forcing gateway re-resolution to restore path visibility.