chore(vertex_ai): translate chunk error-check comments to english

- Translate docstring and inline comments for the mid-stream error
  detection added in PR #23711 so the codebase stays English-only.
- Apply Black (23.12) formatting on the corresponding regression test.

litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py

@@ -3141,7 +3141,7 @@ class ModelResponseIterator:
 
     @staticmethod
     def _check_streaming_error(chunk: dict) -> None:
-        """检测流式响应中内嵌的错误（如 429 RESOURCE_EXHAUSTED），有错误时抛出 VertexAIError。"""
+        """Detect embedded errors (e.g. 429 RESOURCE_EXHAUSTED) in streaming chunks and raise VertexAIError."""
         if "error" not in chunk:
             return
         error_data = chunk["error"]
@@ -3276,8 +3276,8 @@ class ModelResponseIterator:
         try:
             verbose_logger.debug(f"RAW GEMINI CHUNK: {chunk}")
 
-            # 检测流式响应中内嵌的错误（如 429 RESOURCE_EXHAUSTED）。
-            # 这类错误以 HTTP 200 返回，但 SSE body 中包含 error JSON。
+            # Detect mid-stream error chunks (e.g. 429 RESOURCE_EXHAUSTED).
+            # Vertex AI can return errors as HTTP 200 but with an "error" field in the SSE body.
             self._check_streaming_error(chunk)
 
             from litellm.types.utils import ModelResponseStream

tests/test_litellm/llms/vertex_ai/gemini/test_vertex_and_google_ai_studio_gemini.py

@@ -4466,7 +4466,7 @@ def test_chunk_parser_raises_on_string_error_code():
         ModelResponseIterator,
     )
 
-    # code 字段为字符串类型 "429" 而非 int
+    # code field is a string "429" rather than an int
     error_chunk = {
         "error": {
             "code": "429",
@@ -4575,7 +4575,9 @@ def test_mid_stream_429_error_raises_during_iteration():
                 results.append(chunk)
 
     # Verify: received normal chunks before the error
-    assert len(results) >= 1, "Should have received at least 1 normal chunk before the error"
+    assert (
+        len(results) >= 1
+    ), "Should have received at least 1 normal chunk before the error"
 
     # Verify: 429 error is properly raised
     assert exc_info.value.status_code == 429