fix(native-api): 修复批量嵌入子请求网络异常未捕获导致 500 的问题;

在 _vertex_batch_embed._single 中为 client.send() 添加与 _vertex_single_embed
一致的 httpx 网络异常处理（TimeoutException/ConnectError/ReadError/RemoteProtocolError），
确保子请求超时或连接失败时返回 502 而非触发未处理异常。

🤖 Generated with [Claude Code](https://github.com/claude), [CodeX](https://openai.com), [Gemini](https://github.com/apps/gemini-code-assist)
Co-Authored-By: Aurelius Huang<threefish.ai@gmail.com>

Author: ThreeFish-AI

src/coding/proxy/native_api/handler.py

@@ -553,7 +553,15 @@ async def _single(req_body: dict) -> tuple[dict, int]:
                 content=vertex_body,
                 headers=upstream_headers,
             )
-            resp = await client.send(req, stream=False)
+            try:
+                resp = await client.send(req, stream=False)
+            except (
+                httpx.TimeoutException,
+                httpx.ConnectError,
+                httpx.ReadError,
+                httpx.RemoteProtocolError,
+            ) as exc:
+                return {"error": {"message": f"upstream unreachable: {exc}"}}, 502
             try:
                 return resp.json(), resp.status_code
             except Exception: