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_models/available_source.rs

@@ -1,9 +1,18 @@
-use super::*;
+use super::super::build_admin_provider_available_source_models_payload;
+use crate::gateway::handlers::admin_provider_available_source_models_path;
+use crate::gateway::{AppState, GatewayControlDecision, GatewayError, GatewayPublicRequestContext};
+use axum::{
+    body::{Body, Bytes},
+    http,
+    response::{IntoResponse, Response},
+    Json,
+};
+use serde_json::json;
 
 pub(super) async fn maybe_handle(
     state: &AppState,
     request_context: &GatewayPublicRequestContext,
-    _request_body: Option<&axum::body::Bytes>,
+    _request_body: Option<&Bytes>,
     decision: &GatewayControlDecision,
 ) -> Result<Option<Response<Body>>, GatewayError> {
     if decision.route_family.as_deref() == Some("provider_models_manage")