fix(agent/agentssh): make X11 max port configurable to fix test timeout (#22840)

TestServer_X11_EvictionLRU was timing out under -race because it created
190 sequential SSH shell sessions (~0.55s each = ~105s), exceeding the
90s test timeout. The session count was derived from the production
X11MaxPort constant (6200).

Add a configurable X11MaxPort field to Config so the test can use a
small port range (5 ports instead of 190). This reduces the number of
sessions from 190 to 4, completing in ~3.8s under -race.

agent/agentssh/agentssh.go

@@ -110,6 +110,11 @@ type Config struct {
 	// X11DisplayOffset is the offset to add to the X11 display number.
 	// Default is 10.
 	X11DisplayOffset *int
+	// X11MaxPort overrides the highest port used for X11 forwarding
+	// listeners. Defaults to X11MaxPort (6200). Useful in tests
+	// to shrink the port range and reduce the number of sessions
+	// required.
+	X11MaxPort *int
 	// BlockFileTransfer restricts use of file transfer applications.
 	BlockFileTransfer bool
 	// ReportConnection.
@@ -158,6 +163,10 @@ func NewServer(ctx context.Context, logger slog.Logger, prometheusRegistry *prom
 		offset := X11DefaultDisplayOffset
 		config.X11DisplayOffset = &offset
 	}
+	if config.X11MaxPort == nil {
+		maxPort := X11MaxPort
+		config.X11MaxPort = &maxPort
+	}
 	if config.UpdateEnv == nil {
 		config.UpdateEnv = func(current []string) ([]string, error) { return current, nil }
 	}
@@ -201,6 +210,7 @@ func NewServer(ctx context.Context, logger slog.Logger, prometheusRegistry *prom
 			x11HandlerErrors: metrics.x11HandlerErrors,
 			fs:               fs,
 			displayOffset:    *config.X11DisplayOffset,
+			maxPort:          *config.X11MaxPort,
 			sessions:         make(map[*x11Session]struct{}),
 			connections:      make(map[net.Conn]struct{}),
 			network: func() X11Network {

agent/agentssh/x11.go

@@ -57,6 +57,7 @@ type x11Forwarder struct {
 	x11HandlerErrors *prometheus.CounterVec
 	fs               afero.Fs
 	displayOffset    int
+	maxPort          int
 
 	// network creates X11 listener sockets. Defaults to osNet{}.
 	network X11Network
@@ -314,7 +315,7 @@ func (x *x11Forwarder) evictLeastRecentlyUsedSession() {
 // the next available port starting from X11StartPort and displayOffset.
 func (x *x11Forwarder) createX11Listener(ctx context.Context) (ln net.Listener, display int, err error) {
 	// Look for an open port to listen on.
-	for port := X11StartPort + x.displayOffset; port <= X11MaxPort; port++ {
+	for port := X11StartPort + x.displayOffset; port <= x.maxPort; port++ {
 		if ctx.Err() != nil {
 			return nil, -1, ctx.Err()
 		}

agent/agentssh/x11_test.go

@@ -142,8 +142,13 @@ func TestServer_X11_EvictionLRU(t *testing.T) {
 	// Use in-process networking for X11 forwarding.
 	inproc := testutil.NewInProcNet()
 
+	// Limit port range so we only need a handful of sessions to fill it
+	// (the default 190 ports may easily timeout or conflict with other
+	// ports on the system).
+	maxPort := agentssh.X11StartPort + agentssh.X11DefaultDisplayOffset + 5
 	cfg := &agentssh.Config{
-		X11Net: inproc,
+		X11Net:     inproc,
+		X11MaxPort: &maxPort,
 	}
 
 	s, err := agentssh.NewServer(ctx, logger, prometheus.NewRegistry(), fs, agentexec.DefaultExecer, cfg)
@@ -172,7 +177,7 @@ func TestServer_X11_EvictionLRU(t *testing.T) {
 	// configured port range.
 
 	startPort := agentssh.X11StartPort + agentssh.X11DefaultDisplayOffset
-	maxSessions := agentssh.X11MaxPort - startPort + 1 - 1 // -1 for the blocked port
+	maxSessions := maxPort - startPort + 1 - 1 // -1 for the blocked port
 	require.Greater(t, maxSessions, 0, "expected a positive maxSessions value")
 
 	// shellSession holds references to the session and its standard streams so