fix: add missing cost tracker methods + guard logger against null wpdb

- Add `would_exceed_budget(float)` to pre-check budget before API calls
- Add `log_image_generation()` for image pipeline cost logging
- Guard Logger::log() against null $wpdb in unit test environments
- Both methods called by image pipeline (commit 1b) but were missing

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

includes/core/class-logger.php

@@ -126,6 +126,11 @@ class PRAutoBlogger_Logger {
 		}
 
 		global $wpdb;
+		if ( null === $wpdb ) {
+			// No database available (unit test environment). PHP error_log
+			// was already called above for warnings/errors; silently skip DB write.
+			return;
+		}
 		$table = $wpdb->prefix . 'prautoblogger_event_log';
 
 		// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery