refactor: 大规模模块拆分与代码精简，新增 ai-pipeline/data-contracts 独立 crate

- 新增 aether-ai-pipeline 和 aether-data-contracts crate，将 pipeline 逻辑与数据契约从 gateway 中解耦
- 重构 admin handlers：拆分单体模块为 auth/billing/endpoint/features/model/observability/provider/system 等独立子模块
- 合并 chat/cli 重复代码路径：精简 conversion、finalize、planner 中的 sync/chat/cli 分支
- 重构 scheduler/executor/data 层，引入 facade 模式降低模块间耦合
- 移除冗余的 intent 模块，将 plan_fallback/policy/stream_path/sync_path 迁移至 executor
- 前端适配：调整 admin API 调用和 provider 模型测试对话框

apps/aether-gateway/src/handlers/shared/request_utils.rs

@@ -4,13 +4,14 @@ use crate::constants::{
 };
 use crate::control::GatewayControlDecision;
 use crate::control::GatewayPublicRequestContext;
-use crate::handlers::OFFICIAL_EXTERNAL_MODEL_PROVIDERS;
 use crate::headers::header_value_str;
 use crate::tunnel::TUNNEL_ROUTE_FAMILY;
 use axum::http::{self, HeaderName};
 use chrono::{SecondsFormat, Utc};
 use url::form_urlencoded;
 
+use super::OFFICIAL_EXTERNAL_MODEL_PROVIDERS;
+
 pub(crate) fn rust_auth_terminates_provider_credentials(
     decision: Option<&GatewayControlDecision>,
 ) -> bool {