[client/management/signal/relay] Update go.mod to use Go 1.24.10 and upgrade x/crypto dependencies (#4828)

Upgrade Go toolchain and golang.org/x/* deps to 1.24.10, standardize GitHub Actions to derive Go version from go.mod and adjust checkout ordering, raise WASM size limit to 55 MB, update FreeBSD tarball and gomobile refs, fix a few format-string/logging calls, treat usernames ending with $ as system accounts, and add Windows tests.

client/ssh/testutil/user_helpers.go

@@ -72,7 +72,8 @@ func IsSystemAccount(username string) bool {
 			return true
 		}
 	}
-	return false
+
+	return strings.HasSuffix(username, "$")
 }
 
 // RegisterTestUserCleanup registers a test user for cleanup

client/ssh/testutil/user_helpers_test.go

@@ -0,0 +1,115 @@
+package testutil
+
+import (
+	"os/user"
+	"runtime"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestUserCurrentBehavior validates user.Current() behavior on Windows.
+// When running as SYSTEM on a domain-joined machine, user.Current() returns:
+// - Username: Computer account name (e.g., "DOMAIN\MACHINE$")
+// - SID: SYSTEM SID (S-1-5-18)
+func TestUserCurrentBehavior(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("Windows-specific test")
+	}
+
+	currentUser, err := user.Current()
+	require.NoError(t, err, "Should be able to get current user")
+
+	t.Logf("Current user - Username: %s, SID: %s", currentUser.Username, currentUser.Uid)
+
+	// When running as SYSTEM, validate expected behavior
+	if currentUser.Uid == "S-1-5-18" {
+		t.Run("SYSTEM_account_behavior", func(t *testing.T) {
+			// SID must be S-1-5-18 for SYSTEM
+			require.Equal(t, "S-1-5-18", currentUser.Uid,
+				"SYSTEM account must have SID S-1-5-18")
+
+			// Username can be either "NT AUTHORITY\SYSTEM" (standalone)
+			// or "DOMAIN\MACHINE$" (domain-joined)
+			username := currentUser.Username
+			isNTAuthority := strings.Contains(strings.ToUpper(username), "NT AUTHORITY")
+			isComputerAccount := strings.HasSuffix(username, "$")
+
+			assert.True(t, isNTAuthority || isComputerAccount,
+				"Username should be either 'NT AUTHORITY\\SYSTEM' or computer account (ending with $), got: %s",
+				username)
+
+			if isComputerAccount {
+				t.Logf("SYSTEM as computer account: %s", username)
+			} else if isNTAuthority {
+				t.Logf("SYSTEM as NT AUTHORITY\\SYSTEM")
+			}
+		})
+	}
+
+	// Validate that IsSystemAccount correctly identifies system accounts
+	t.Run("IsSystemAccount_validation", func(t *testing.T) {
+		// Test with current user if it's a system account
+		if currentUser.Uid == "S-1-5-18" || // SYSTEM
+			currentUser.Uid == "S-1-5-19" || // LOCAL SERVICE
+			currentUser.Uid == "S-1-5-20" { // NETWORK SERVICE
+
+			result := IsSystemAccount(currentUser.Username)
+			assert.True(t, result,
+				"IsSystemAccount should recognize system account: %s (SID: %s)",
+				currentUser.Username, currentUser.Uid)
+		}
+
+		// Test explicit cases
+		testCases := []struct {
+			username string
+			expected bool
+			reason   string
+		}{
+			{"NT AUTHORITY\\SYSTEM", true, "NT AUTHORITY\\SYSTEM"},
+			{"system", true, "system"},
+			{"SYSTEM", true, "SYSTEM (case insensitive)"},
+			{"NT AUTHORITY\\LOCAL SERVICE", true, "LOCAL SERVICE"},
+			{"NT AUTHORITY\\NETWORK SERVICE", true, "NETWORK SERVICE"},
+			{"DOMAIN\\MACHINE$", true, "computer account (ends with $)"},
+			{"WORKGROUP\\WIN2K19-C2$", true, "computer account (ends with $)"},
+			{"Administrator", false, "Administrator is not a system account"},
+			{"alice", false, "regular user"},
+			{"DOMAIN\\alice", false, "domain user"},
+		}
+
+		for _, tc := range testCases {
+			t.Run(tc.username, func(t *testing.T) {
+				result := IsSystemAccount(tc.username)
+				assert.Equal(t, tc.expected, result,
+					"IsSystemAccount(%q) should be %v because: %s",
+					tc.username, tc.expected, tc.reason)
+			})
+		}
+	})
+}
+
+// TestComputerAccountDetection validates computer account detection.
+func TestComputerAccountDetection(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("Windows-specific test")
+	}
+
+	computerAccounts := []string{
+		"MACHINE$",
+		"WIN2K19-C2$",
+		"DOMAIN\\MACHINE$",
+		"WORKGROUP\\SERVER$",
+		"server.domain.com$",
+	}
+
+	for _, account := range computerAccounts {
+		t.Run(account, func(t *testing.T) {
+			result := IsSystemAccount(account)
+			assert.True(t, result,
+				"Computer account %q should be recognized as system account", account)
+		})
+	}
+}