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

apps/aether-gateway/src/execution_runtime/sync/execution/policy.rs

@@ -0,0 +1,36 @@
+use std::collections::BTreeMap;
+
+use aether_contracts::ExecutionResult;
+use base64::Engine as _;
+
+use crate::gateway::GatewayError;
+
+type DecodedBody = (Vec<u8>, Option<serde_json::Value>, Option<String>);
+
+pub(super) fn decode_execution_result_body(
+    result: &ExecutionResult,
+    headers: &mut BTreeMap<String, String>,
+) -> Result<DecodedBody, GatewayError> {
+    let Some(body) = result.body.as_ref() else {
+        return Ok((Vec::new(), None, None));
+    };
+
+    if let Some(json_body) = body.json_body.clone() {
+        headers
+            .entry("content-type".to_string())
+            .or_insert_with(|| "application/json".to_string());
+        let bytes = serde_json::to_vec(&json_body)
+            .map_err(|err| GatewayError::Internal(err.to_string()))?;
+        headers.insert("content-length".to_string(), bytes.len().to_string());
+        return Ok((bytes, Some(json_body), None));
+    }
+
+    if let Some(body_bytes_b64) = body.body_bytes_b64.clone() {
+        let bytes = base64::engine::general_purpose::STANDARD
+            .decode(&body_bytes_b64)
+            .map_err(|err| GatewayError::Internal(err.to_string()))?;
+        return Ok((bytes, None, Some(body_bytes_b64)));
+    }
+
+    Ok((Vec::new(), None, None))
+}