🐛fix: Use feed publication date as post_date on import

Extracted the wp_insert_post/wp_update_post argument construction into a
private build_postarr() helper so the post_date/post_date_gmt derivation
from the feed timestamp can be exercised in isolation. Added smoke tests
that pin down the create-path behavior (feed timestamp becomes post_date)
and the fallback path (no feed date leaves post_date unset for WordPress
to fill in at import time). Added minimal get_date_from_gmt and
get_gmt_from_date stubs to the test bootstrap so the new tests run
without WordPress loaded.

tests/bootstrap.php

@@ -31,5 +31,29 @@ function wp_parse_args( $args, $defaults = array() ) {
 	return array_merge( $defaults, $args );
 }
 
+/**
+ * Minimal get_date_from_gmt stub matching WordPress's MySQL-string behavior.
+ *
+ * @param string $date_string GMT timestamp in MySQL date format.
+ * @return string Timestamp converted to the site's local timezone.
+ */
+function get_date_from_gmt( $date_string ) {
+	return get_gmt_from_date( $date_string );
+}
+
+/**
+ * Minimal get_gmt_from_date stub: the inverse of get_date_from_gmt.
+ *
+ * @param string $date_string Local timestamp in MySQL date format.
+ * @return string Timestamp converted to GMT (UTC).
+ */
+function get_gmt_from_date( $date_string ) {
+	$timestamp = strtotime( $date_string . ' UTC' );
+	if ( false === $timestamp ) {
+		return $date_string;
+	}
+	return gmdate( 'Y-m-d H:i:s', $timestamp );
+}
+
 require_once dirname( __DIR__ ) . '/src/Options.php';
 require_once dirname( __DIR__ ) . '/src/Import/Importer.php';