fix: wire Image B source data — was hardcoded null, never generated

Root cause: both attach_generated_images() in post-assembler and the
retroactive handler in executor passed null as source_data to the image
pipeline. The pipeline's Image B branch requires non-null source_data
to fire, so Image B was silently skipped on every article since the
feature was built.

Fix: added get_source_data_for_image() to Source_Collector to retrieve
the original Reddit post title/content by source_ids. Post-assembler
now pulls source_ids from the Article_Idea; executor pulls them from
the _prautoblogger_source_ids post meta saved by the publisher.

Also added a regression test proving Image B is skipped when source_data
is null, so this handoff gap will be caught if re-introduced.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/core/class-source-collector.php

@@ -57,6 +57,47 @@ class PRAutoBlogger_Source_Collector {
 		return $total_inserted;
 	}
 
+	/**
+	 * Retrieve source data rows by their database IDs.
+	 *
+	 * Used by the image pipeline to build Image B's source-driven prompt from
+	 * the source_ids stored on the Article_Idea. Returns the first matching
+	 * row's title and content; these map directly to what
+	 * Image_Prompt_Builder::build_source_prompt() expects.
+	 *
+	 * Side effects: one SELECT query.
+	 *
+	 * @param int[] $ids Database row IDs from the source_data table.
+	 * @return array{title: string, selftext: string}|null First source row, or null if none found.
+	 */
+	public function get_source_data_for_image( array $ids ): ?array {
+		if ( empty( $ids ) ) {
+			return null;
+		}
+
+		global $wpdb;
+		$table        = $wpdb->prefix . 'prautoblogger_source_data';
+		$placeholders = implode( ',', array_fill( 0, count( $ids ), '%d' ) );
+
+		// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.PreparedSQLPlaceholders.UnfinishedPrepare
+		$row = $wpdb->get_row(
+			$wpdb->prepare(
+				"SELECT title, content, subreddit FROM {$table} WHERE id IN ({$placeholders}) ORDER BY score DESC LIMIT 1",
+				...$ids
+			),
+			ARRAY_A
+		);
+
+		if ( null === $row ) {
+			return null;
+		}
+
+		return [
+			'title'    => $row['title'] ?? 'Reddit Discussion',
+			'selftext' => $row['content'] ?? '',
+		];
+	}
+
 	/**
 	 * Store collected source data in the database.
 	 *