feat: 引入 Rust executor/gateway sidecar 及 Python 侧双后端适配

- 新增 Rust workspace crates: aether-contracts, aether-executor, aether-gateway
- aether-executor: 支持 Unix Socket/TCP 双传输模式，处理同步/流式上游请求
- aether-gateway: 作为本地主入口代理，集成 /api/internal/gateway/resolve 认证预解析
- Python 侧新增 ExecutionPlan 契约和 RustExecutorClient，各 handler 支持
  executor_backend=rust 时将可序列化请求转发给 Rust executor 执行
- 重构 dev.sh 支持 executor/gateway 进程编排与生命周期管理
- 新增 internal gateway 路由，提供 resolve/passthrough 端点
- handler 层(chat/cli/video/endpoint_checker 等)全面适配 Rust executor 回退逻辑
- pipeline 层支持 trusted auth context 跳过重复认证
- 新增 Rust CI workflow 及对应测试用例

crates/aether-contracts/src/error.rs

@@ -0,0 +1,41 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum ExecutionErrorKind {
+    ConnectTimeout,
+    FirstByteTimeout,
+    ReadTimeout,
+    Upstream4xx,
+    Upstream5xx,
+    TlsError,
+    ProxyError,
+    ProtocolError,
+    Cancelled,
+    Internal,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum ExecutionPhase {
+    Connect,
+    Handshake,
+    Write,
+    FirstByte,
+    StreamRead,
+    Decode,
+    Finalize,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+pub struct ExecutionError {
+    pub kind: ExecutionErrorKind,
+    pub phase: ExecutionPhase,
+    pub message: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub upstream_status: Option<u16>,
+    #[serde(default)]
+    pub retryable: bool,
+    #[serde(default)]
+    pub failover_recommended: bool,
+}