refactor: 移除 Python upstream 依赖，清理全部 legacy/Python 兼容层

- 移除 upstream_base_url 参数及 AETHER_GATEWAY_UPSTREAM 环境变量，gateway 不再需要指向 Python 宿主
- 删除所有 LEGACY_*/PYTHON_* 常量、路由组、header 定义及 sunset/phaseout 机制
- 将 legacy_gateway_bridge 重命名为 internal_gateway，executor 相关命名统一为 execution_runtime
- dev.sh 新增 Postgres/Redis 预检查，移除 upstream 相关启动参数和提示
- 新增 ai_public 路由处理器
- 全量适配 handler、test、state、control 等模块的命名和接口变更

apps/aether-gateway/src/gateway/tests/ai_execute/sync/mod.rs

@@ -1,4 +1,27 @@
-use super::*;
+use std::convert::Infallible;
+use std::sync::{Arc, Mutex};
+
+use axum::body::{to_bytes, Body, Bytes};
+use axum::response::Response;
+use axum::routing::any;
+use axum::{extract::Request, Json, Router};
+use http::header::{HeaderName, HeaderValue};
+use http::StatusCode;
+use serde_json::json;
+
+use crate::gateway::constants::{
+    CONTROL_EXECUTED_HEADER, CONTROL_EXECUTE_FALLBACK_HEADER, DEPENDENCY_REASON_HEADER,
+    EXECUTION_PATH_EXECUTION_RUNTIME_STREAM, EXECUTION_PATH_EXECUTION_RUNTIME_SYNC,
+    EXECUTION_PATH_HEADER, EXECUTION_PATH_LOCAL_EXECUTION_RUNTIME_MISS,
+    LOCAL_EXECUTION_RUNTIME_MISS_REASON_HEADER, TRACE_ID_HEADER,
+};
+
+use super::{
+    build_router, build_router_with_execution_runtime_override, build_router_with_state,
+    build_state_with_execution_runtime_override, start_server, wait_until, AppState,
+    FrontdoorCorsConfig, FrontdoorUserRpmConfig, GatewayFallbackMetricKind,
+    GatewayFallbackReason, UsageRuntimeConfig, VideoTaskTruthSourceMode,
+};
 
 mod chat;
 mod claude;