[relay] evict foreign client cache on disconnect (#6015)

* [relay] evict foreign client cache on disconnect

When a foreign relay's TCP connection drops, the manager's
onServerDisconnected handler only triggered reconnect logic for the
home server; the disconnected foreign entry stayed in the relayClients
cache. Subsequent OpenConn calls reused the closed client until the
60-second cleanup tick evicted it, breaking peer connectivity through
that relay for up to a minute.

Evict the foreign entry from the cache on disconnect so the next
OpenConn dials a fresh client.

Also:
- Make the reconnect backoff cap configurable via WithMaxBackoffInterval
  ManagerOption; the previous hard-coded 60s constant forced
  TestAutoReconnect to sleep ~61s. Test now polls Ready() and finishes
  in ~2s.
- Add NB_HOME_RELAY_SERVERS env var that overrides the relay URL list
  received from management, so a peer can be pinned to a specific home
  relay (used by the netbird-conn-lab Edge 4 reproducer).

* [client] treat empty NB_HOME_RELAY_SERVERS as unset

Returning (urls=[], ok=true) when the env var contained only separators or
whitespace caused callers to wipe the mgmt-provided relay list, leaving the
peer with no relays. Treat a parsed-empty result the same as an unset env.

client/internal/engine.go

@@ -944,7 +944,12 @@ func (e *Engine) handleRelayUpdate(update *mgmProto.RelayConfig) error {
 			return fmt.Errorf("update relay token: %w", err)
 		}
 
-		e.relayManager.UpdateServerURLs(update.Urls)
+		urls := update.Urls
+		if override, ok := peer.OverrideRelayURLs(); ok {
+			log.Infof("overriding relay URLs from %s: %v", peer.EnvKeyNBHomeRelayServers, override)
+			urls = override
+		}
+		e.relayManager.UpdateServerURLs(urls)
 
 		// Just in case the agent started with an MGM server where the relay was disabled but was later enabled.
 		// We can ignore all errors because the guard will manage the reconnection retries.