fix: variant update now sets attribute values and default variant on WC

- Update path sends attributes per variation (was missing, causing
  "Any COLOR..." on WC)
- Sets default_attributes on parent product (first variant's values)
- Better API error logging with response body

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

fusion-woo-odoo/fusion_woocommerce/lib/woo_api_client.py

@@ -39,6 +39,11 @@ class WooApiClient:
                     params=params,
                     timeout=self.timeout,
                 )
+                if response.status_code >= 400:
+                    _logger.error(
+                        "WC API %s %s returned %s: %s",
+                        method, endpoint, response.status_code, response.text[:500],
+                    )
                 response.raise_for_status()
                 return response.json()
             except Exception as exc: