[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/connect.go

@@ -333,6 +333,10 @@ func (c *ConnectClient) run(mobileDependency MobileDependency, runningChan chan
 		c.statusRecorder.MarkSignalConnected()
 
 		relayURLs, token := parseRelayInfo(loginResp)
+		if override, ok := peer.OverrideRelayURLs(); ok {
+			log.Infof("overriding relay URLs from %s: %v", peer.EnvKeyNBHomeRelayServers, override)
+			relayURLs = override
+		}
 		peerConfig := loginResp.GetPeerConfig()
 
 		engineConfig, err := createEngineConfig(myPrivateKey, c.config, peerConfig, logPath)