Bug: Apply SELinux context after binary replacement (#1678)

- Move SELinux context handling to internal/ghupdate for reuse
- Make chcon a true fallback (only runs if semanage/restorecon unavailable)
- Handle existing semanage rules with -m (modify) after -a (add) fails
- Apply SELinux handling to both agent and hub updates
- Add tests with proper skip behavior for SELinux systems

---------

Co-authored-by: henrygd <hank@henrygd.me>

internal/ghupdate/selinux_test.go

@@ -0,0 +1,53 @@
+package ghupdate
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+func TestHandleSELinuxContext_NoSELinux(t *testing.T) {
+	// Skip on SELinux systems - this test is for non-SELinux behavior
+	if _, err := exec.LookPath("getenforce"); err == nil {
+		t.Skip("skipping on SELinux-enabled system")
+	}
+
+	// On systems without SELinux, getenforce will fail and the function
+	// should return nil without error
+	tempFile := filepath.Join(t.TempDir(), "test-binary")
+	if err := os.WriteFile(tempFile, []byte("test"), 0755); err != nil {
+		t.Fatalf("failed to create temp file: %v", err)
+	}
+
+	err := HandleSELinuxContext(tempFile)
+	if err != nil {
+		t.Errorf("HandleSELinuxContext() on non-SELinux system returned error: %v", err)
+	}
+}
+
+func TestHandleSELinuxContext_InvalidPath(t *testing.T) {
+	// Skip on SELinux systems - this test is for non-SELinux behavior
+	if _, err := exec.LookPath("getenforce"); err == nil {
+		t.Skip("skipping on SELinux-enabled system")
+	}
+
+	// On non-SELinux systems, getenforce fails early so even invalid paths succeed
+	err := HandleSELinuxContext("/nonexistent/path/binary")
+	if err != nil {
+		t.Errorf("HandleSELinuxContext() with invalid path on non-SELinux system returned error: %v", err)
+	}
+}
+
+func TestTrySemanageRestorecon_NoTools(t *testing.T) {
+	// Skip if semanage is available (we don't want to modify system SELinux policy)
+	if _, err := exec.LookPath("semanage"); err == nil {
+		t.Skip("skipping on system with semanage available")
+	}
+
+	// Should return false when semanage is not available
+	result := trySemanageRestorecon("/some/path")
+	if result {
+		t.Error("trySemanageRestorecon() returned true when semanage is not available")
+	}
+}