mbproxy: add keepalive / connection monitoring

The DL205/DL260 ECOM emits no TCP keepalives, so an idle backend socket
can be silently dropped by a middlebox (switch, firewall, NAT) after
2-5 minutes. Enable OS SO_KEEPALIVE on backend and accepted upstream
sockets, and drive a periodic synthetic FC03 heartbeat on each idle
backend socket so a dead path is detected before a real client request
hits it. Controlled by Connection.Keepalive (ON by default).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/tests/Mbproxy.Tests/Admin/StatusHtmlRendererTests.cs

@@ -53,7 +53,9 @@ public sealed class StatusHtmlRendererTests
                 CoalescedHitCount: 0, CoalescedMissCount: 0,
                 CoalescedResponseToDeadUpstream: 0,
                 CacheHitCount: 0, CacheMissCount: 0,
-                CacheInvalidations: 0, CacheEntryCount: 0, CacheBytes: 0),
+                CacheInvalidations: 0, CacheEntryCount: 0, CacheBytes: 0,
+                BackendHeartbeatsSent: 0, BackendHeartbeatsFailed: 0,
+                BackendIdleDisconnects: 0),
             Bytes:      new PlcBytesStatus(1024, 2048));
     }