refactor(gateway): 重构格式转换候选资格检查并优化测试基础设施

- 将 format_conversion_disabled 的过滤提前到候选遴选阶段，替代原来的 skip_reason 标记机制
- 为测试添加 execution_runtime_sync_override 直接注入支持，避免启动额外 HTTP 服务器
- 将 submit_terminal_event 改为 async record_terminal_event 确保失败用量事件可靠入库
- 新增 test_support 模块封装可重试的 loopback 端口绑定逻辑
- 前端：poolTrace 将 skipped 从隐藏状态移除，新增 buildPoolParticipatedCandidates 统一新旧链路逻辑，并将 skipped 状态色改为 foreground

apps/aether-gateway/src/state/testing.rs

@@ -1,6 +1,7 @@
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex as StdMutex};
 
+use aether_contracts::{ExecutionPlan, ExecutionResult};
 use aether_data_contracts::repository::candidates::RequestCandidateReadRepository;
 use aether_data_contracts::repository::provider_catalog::ProviderCatalogReadRepository;
 use aether_data_contracts::repository::usage::{UsageReadRepository, UsageRepository};
@@ -170,6 +171,22 @@ impl AppState {
         self
     }
 
+    pub(crate) fn with_execution_runtime_sync_override_for_tests<F>(
+        mut self,
+        override_fn: F,
+    ) -> Self
+    where
+        F: Fn(&ExecutionPlan) -> Result<ExecutionResult, crate::GatewayError>
+            + Send
+            + Sync
+            + 'static,
+    {
+        self.execution_runtime_sync_override = Some(super::app::TestExecutionRuntimeSyncOverride(
+            Arc::new(override_fn),
+        ));
+        self
+    }
+
     pub(crate) fn with_oauth_refresh_coordinator_for_tests(
         mut self,
         coordinator: provider_transport::LocalOAuthRefreshCoordinator,