refactor: 大规模模块拆分与重组，新增 aether-admin crate

- 新建独立 aether-admin crate 承载 admin 相关共享契约与纯辅助函数
- 拆分 ai_pipeline 下 kiro/private_envelope/conversion/planner 等大文件为子模块目录
- 重组 admin handlers 各业务域（billing/oauth/provider/system/users 等）为目录结构，移除 shared.rs/builders.rs 等反模式
- 移除 ai_pipeline runtime adapters 旧实现（claude/openai/gemini/kiro/vertex/antigravity 等），改由 provider transport 统一承载
- 移除 control_facade/execution_facade/auth_snapshot_facade 等冗余 facade 层
- 拆分 query/billing 与 query/monitoring 模块、state/runtime/payments 与 security 模块
- 扩展架构测试覆盖 admin_billing/admin_model/admin_users 等新模块
- 删除 docs/architecture/refactor-execution-plan.md 已完成的执行计划文档

apps/aether-gateway/src/handlers/admin/users/mod.rs

@@ -1,11 +1,10 @@
-use crate::control::GatewayPublicRequestContext;
-use crate::{AppState, GatewayError};
-use axum::{body::Body, response::Response};
+use crate::handlers::admin::request::{AdminRouteRequest, AdminRouteResult};
 
 const ADMIN_USERS_DATA_UNAVAILABLE_DETAIL: &str = "Admin user management data unavailable";
 
 mod api_keys;
 mod lifecycle;
+mod route_seam;
 mod routes;
 mod sessions;
 mod shared;
@@ -40,10 +39,7 @@ use self::shared::{
 pub(crate) use self::shared::{normalize_admin_user_api_formats, normalize_admin_user_string_list};
 
 pub(crate) async fn maybe_build_local_admin_users_response(
-    state: &AppState,
-    request_context: &GatewayPublicRequestContext,
-    request_body: Option<&axum::body::Bytes>,
-) -> Result<Option<Response<Body>>, GatewayError> {
-    routes::maybe_build_local_admin_users_routes_response(state, request_context, request_body)
-        .await
+    request: AdminRouteRequest<'_>,
+) -> AdminRouteResult {
+    route_seam::maybe_build_local_admin_users_response(request).await
 }