Returns a symmetric 2D matrix representing tunnel link status between all appliances in the specified overlay. Each cell [i][j] contains the worst-case status between bidirectional tunnels (src→dst and dst→src). If appliance count exceeds APPLIANCE_TOPOLOGY_LINK_THRESHOLD, the linkInfo array is not populated (null). Supports ETag-based caching - provide If-None-Match header with previous ETag to receive 304 if data unchanged.