refactor: 移除独立 hub/proxy/executor/gateway crate，统一为 gateway tunnel 架构

- 删除 aether-hub、aether-proxy 独立项目及其 Dockerfile/配置
- 删除 crates/aether-executor 和 crates/aether-gateway 全部模块
- 新增 apps/ 目录作为应用入口
- 将 hub 概念重构为 gateway tunnel transport
- 将 executor 重构为 execution runtime
- 新增 tunnel.rs 合约定义和 testkit tunnel/execution_runtime 模块
- 更新 Python 服务层和测试适配新架构命名

crates/aether-testkit/src/execution_runtime.rs

@@ -0,0 +1,55 @@
+use aether_gateway::build_execution_runtime_router_with_request_gates;
+use aether_runtime::DistributedConcurrencyGate;
+
+use crate::server::SpawnedServer;
+
+#[derive(Debug, Clone, Default)]
+pub struct ExecutionRuntimeHarnessConfig {
+    pub max_in_flight_requests: Option<usize>,
+    pub distributed_request_gate: Option<DistributedConcurrencyGate>,
+}
+
+#[derive(Debug)]
+pub struct ExecutionRuntimeHarness {
+    server: SpawnedServer,
+}
+
+impl ExecutionRuntimeHarness {
+    pub async fn start(config: ExecutionRuntimeHarnessConfig) -> Result<Self, String> {
+        Self::start_with_server(config, None).await
+    }
+
+    pub async fn start_on_port(
+        config: ExecutionRuntimeHarnessConfig,
+        port: u16,
+    ) -> Result<Self, String> {
+        Self::start_with_server(config, Some(port)).await
+    }
+
+    async fn start_with_server(
+        config: ExecutionRuntimeHarnessConfig,
+        port: Option<u16>,
+    ) -> Result<Self, String> {
+        let router = build_execution_runtime_router_with_request_gates(
+            config.max_in_flight_requests,
+            config.distributed_request_gate,
+        );
+        let server = match port {
+            Some(port) => SpawnedServer::start_on_port(port, router)
+                .await
+                .map_err(|err| format!("failed to start execution runtime harness: {err}"))?,
+            None => SpawnedServer::start(router)
+                .await
+                .map_err(|err| format!("failed to start execution runtime harness: {err}"))?,
+        };
+        Ok(Self { server })
+    }
+
+    pub fn base_url(&self) -> &str {
+        self.server.base_url()
+    }
+
+    pub fn port(&self) -> u16 {
+        self.server.port()
+    }
+}