fix(vertex_ai): narrow field transformation to contents and systemInstruction

- Restrict recursive camelCase transformation to 'contents' and 'system_instruction' chunks to avoid breaking existing snake_case top-level fields (e.g. in generationConfig or extra_body).
- Rename 'system_instruction' to 'systemInstruction' for Vertex AI REST API compliance.
- Update context caching transformation with the same narrowed logic.

litellm/llms/vertex_ai/context_caching/transformation.py

@@ -14,6 +14,7 @@ from litellm.utils import is_cached_message
 from ..common_utils import get_supports_system_message
 from ..gemini.transformation import (
     _gemini_convert_messages_with_history,
+    _transform_part_to_httpx_format,
     _transform_system_message,
 )
 
@@ -198,8 +199,14 @@ def transform_openai_messages_to_gemini_context_caching(
         data["system_instruction"] = transformed_system_messages
 
     if is_vertex_ai:
-        from ..gemini.transformation import _transform_part_to_httpx_format
-
-        return _transform_part_to_httpx_format(data)  # type: ignore
+        if "contents" in data:
+            data["contents"] = _transform_part_to_httpx_format(
+                {"contents": data["contents"]}, parent_key=None
+            )["contents"]
+        if "system_instruction" in data:
+            data["systemInstruction"] = _transform_part_to_httpx_format(
+                {"system_instruction": data["system_instruction"]}, parent_key=None
+            )["systemInstruction"]
+            del data["system_instruction"]
 
     return data

litellm/llms/vertex_ai/gemini/transformation.py

@@ -827,7 +827,16 @@ def _transform_request_body(  # noqa: PLR0915
         raise e
 
     if custom_llm_provider in ["vertex_ai", "vertex_ai_beta"]:
-        return _transform_part_to_httpx_format(data)  # type: ignore
+        if "contents" in data:
+            data["contents"] = _transform_part_to_httpx_format(
+                {"contents": data["contents"]}, parent_key=None
+            )["contents"]
+        if "system_instruction" in data:
+            data["systemInstruction"] = _transform_part_to_httpx_format(
+                {"system_instruction": data["system_instruction"]}, parent_key=None
+            )["systemInstruction"]
+            del data["system_instruction"]
+
     return data