fix(container): check context before select in Stop to fix flaky test

Stop() now checks ctx.Err() before entering the select block. When a
pre-cancelled context is passed, the select could non-deterministically
choose <-done over <-ctx.Done() if the process had already exited,
causing TestLinuxKitManager_Stop_Good_ContextCancelled to fail on CI.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/container/linuxkit.go

@@ -258,6 +258,12 @@ func (m *LinuxKitManager) Stop(ctx context.Context, id string) error {
 		return nil
 	}
 
+	// Honour already-cancelled contexts before waiting
+	if err := ctx.Err(); err != nil {
+		_ = process.Signal(syscall.SIGKILL)
+		return err
+	}
+
 	// Wait for graceful shutdown with timeout
 	done := make(chan struct{})
 	go func() {