ssh: say why when there is no shell to run

The published container image has no shell, so a session used to end at once with
nothing printed for a command. It now explains that, and what to do about it,
and exits with status 127.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

sshd/run_unix_test.go

@@ -82,3 +82,32 @@ func TestShell(t *testing.T) {
 		t.Fatalf("got %q, %v", out.String(), err)
 	}
 }
+
+// A machine with no shell (the published container image is one) must say so, not just hang up.
+func TestNoShellIsExplained(t *testing.T) {
+	t.Setenv("SHELL", "/nonexistent/shell")
+	for name, usePty := range map[string]bool{"exec": false, "pty shell": true} {
+		s, done := session(t)
+		var stderr, stdout strings.Builder
+		s.Stderr, s.Stdout = &stderr, &stdout
+		var err error
+		if usePty {
+			if err = s.RequestPty("xterm", 24, 80, ssh.TerminalModes{}); err == nil {
+				err = s.Shell()
+				if err == nil {
+					err = s.Wait()
+				}
+			}
+		} else {
+			err = s.Run("true")
+		}
+		done()
+		var ee *ssh.ExitError
+		if !errors.As(err, &ee) || ee.ExitStatus() != 127 {
+			t.Errorf("%s: want exit status 127, got %v", name, err)
+		}
+		if !strings.Contains(stderr.String()+stdout.String(), "no shell to run") {
+			t.Errorf("%s: no explanation (stderr %q, stdout %q)", name, stderr.String(), stdout.String())
+		}
+	}
+}