[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.

shared/relay/client/manager_test.go

@@ -2,6 +2,7 @@ package client
 
 import (
 	"context"
+	"fmt"
 	"testing"
 	"time"
 
@@ -360,7 +361,8 @@ func TestAutoReconnect(t *testing.T) {
 		t.Fatalf("failed to serve manager: %s", err)
 	}
 
-	clientAlice := NewManager(mCtx, toURL(srvCfg), "alice", iface.DefaultMTU)
+	clientAlice := NewManager(mCtx, toURL(srvCfg), "alice", iface.DefaultMTU,
+		WithMaxBackoffInterval(2*time.Second))
 	err = clientAlice.Serve()
 	if err != nil {
 		t.Fatalf("failed to serve manager: %s", err)
@@ -384,7 +386,9 @@ func TestAutoReconnect(t *testing.T) {
 	}
 
 	log.Infof("waiting for reconnection")
-	time.Sleep(reconnectingTimeout + 1*time.Second)
+	if err := waitForReady(ctx, clientAlice, 15*time.Second); err != nil {
+		t.Fatalf("manager did not reconnect: %s", err)
+	}
 
 	log.Infof("reopent the connection")
 	_, err = clientAlice.OpenConn(ctx, ra, "bob")
@@ -393,6 +397,21 @@ func TestAutoReconnect(t *testing.T) {
 	}
 }
 
+func waitForReady(ctx context.Context, m *Manager, timeout time.Duration) error {
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		if m.Ready() {
+			return nil
+		}
+		select {
+		case <-time.After(100 * time.Millisecond):
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+	return fmt.Errorf("manager not ready within %s", timeout)
+}
+
 func TestNotifierDoubleAdd(t *testing.T) {
 	ctx := context.Background()