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/execution_runtime/stream/execution.rs

@@ -24,10 +24,10 @@ use self::execution_failures::{
     build_stream_failure_from_execution_error, build_stream_failure_report,
     handle_prefetch_stream_failure, submit_midstream_stream_failure, StreamFailureReport,
 };
-use crate::ai_pipeline::adaptation::private_envelope::{
-    maybe_build_provider_private_stream_normalizer, normalize_provider_private_report_context,
+use crate::ai_pipeline_api::{
+    maybe_build_provider_private_stream_normalizer, maybe_build_stream_response_rewriter,
+    normalize_provider_private_report_context,
 };
-use crate::ai_pipeline::finalize::maybe_build_stream_response_rewriter;
 use crate::api::response::{
     attach_control_metadata_headers, build_client_response, build_client_response_from_parts,
 };