Reuse SSH connections for Herdr theme sync (#14460)

* Reuse SSH connections for remote theme sync

* Cover remote theme sync connection reuse

test/shell.d/theme-set-herdr-ssh-reuse-test.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+set -euo pipefail
+
+source "$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)/base-test.sh"
+
+if ! command -v sshd >/dev/null; then
+  skip "no SSH server; skipping real theme sync connection reuse"
+  exit 0
+fi
+require_command python3
+require_command ssh
+require_command ssh-keygen
+
+python3 "$SHELL_TEST_DIR/fixtures/theme-set-herdr-ssh-reuse.py"