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

crates/aether-testkit/src/gateway.rs

@@ -44,7 +44,7 @@ impl GatewayHarness {
         config: GatewayHarnessConfig,
         port: Option<u16>,
     ) -> Result<Self, String> {
-        let mut state = AppState::new(config.upstream_base_url)
+        let mut state = AppState::new()
             .map_err(|err| format!("failed to build gateway harness state: {err}"))?;
         if let Some(data_config) = config.data_config {
             state = state