feat(gateway): harden failover and payload handling

Retry pre-response transport failures across candidates with an explicit stop policy, and propagate end-to-end timing into usage records and UI diagnostics.

Remove legacy body, import, cookie, PII, and tunnel replay caps while preserving optional operator-configured gateway limits.

frontend/src/features/providers/components/ProviderDetailDrawer.vue

@@ -1188,6 +1188,7 @@ const hasFailoverRules = computed(() => {
   if (!rules) return false
   return FAILOVER_RULE_ARRAY_KEYS.some(key => (rules[key]?.length || 0) > 0)
     || typeof rules.max_retries === 'number'
+    || rules.stop_on_transport_errors === true
 })
 
 // Provider 级别代理配置状态