perf(2meet-data-optimizer-hivepress-addon): expire object-cache transients

- with an external object cache, a HivePress timeout entry expires when it
  records; the value (written before its timeout) after at most a week — a
  miss only makes HivePress rebuild it. Entries of deleted posts no longer
  accumulate without end
- Verify: HivepressTransientFilterTest TTL test, integration 53, unit 145;
  dev36 Redis TTL on tmdo_hp_transient keys
- PR: PR-099

tests/integration/bootstrap.php

@@ -21,6 +21,14 @@ if ( ! file_exists( $tmdo_core_bootstrap ) ) {
 		. dirname( __DIR__, 3 ) . '/2meet-data-optimizer'
 	);
 }
+// Defined before the core bootstrap (which only adds it when missing) so tests can read the TTL.
+if ( ! function_exists( 'wp_cache_set' ) ) {
+	function wp_cache_set( $key, $value, $group = '', $ttl = 0 ): bool {
+		$GLOBALS['_wp_cache'][ $group ][ $key ]     = $value;
+		$GLOBALS['_wp_cache_ttl'][ $group ][ $key ] = (int) $ttl;
+		return true;
+	}
+}
 require_once $tmdo_core_bootstrap;
 
 // External object cache switch for the transient filter tests (Redis on real sites).