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/handlers/admin/provider_oauth/dispatch/start.rs

@@ -1,4 +1,19 @@
-use super::*;
+use super::super::provider_oauth_refresh::build_internal_control_error_response;
+use super::super::provider_oauth_state::{
+    admin_provider_oauth_template, build_provider_oauth_start_response,
+    generate_provider_oauth_pkce_verifier, is_fixed_provider_type_for_provider_oauth,
+    provider_oauth_pkce_s256, save_provider_oauth_state,
+};
+use crate::gateway::handlers::{
+    admin_provider_oauth_start_key_id, admin_provider_oauth_start_provider_id,
+};
+use crate::gateway::{AppState, GatewayError, GatewayPublicRequestContext};
+use axum::{
+    body::Body,
+    http,
+    response::{IntoResponse, Response},
+    Json,
+};
 
 pub(super) async fn handle_admin_provider_oauth_start_key(
     state: &AppState,