fix: preserve cache_control on /v1/responses content blocks

_transform_responses_api_content_to_chat_completion_content rebuilt
content blocks with only type+text, dropping cache_control. The
Anthropic adapter (add_cache_control_to_content) never sees it, so
cache is never seeded for /v1/responses.

Fix: copy cache_control from the source item when present (mirrors
the existing pattern for tools).

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: lukesorvikDO

litellm/responses/litellm_completion_transformation/transformation.py

@@ -1286,14 +1286,15 @@ def _transform_responses_api_content_to_chat_completion_content(
                         text_value = item.get("text")
                         if text_value is None:
                             continue
-                        content_list.append(
-                            {
-                                "type": LiteLLMCompletionResponsesConfig._get_chat_completion_request_content_type(
-                                    item.get("type") or "text"
-                                ),
-                                "text": text_value,
-                            }
-                        )
+                        content_block: Dict[str, Any] = {
+                            "type": LiteLLMCompletionResponsesConfig._get_chat_completion_request_content_type(
+                                item.get("type") or "text"
+                            ),
+                            "text": text_value,
+                        }
+                        if item.get("cache_control"):
+                            content_block["cache_control"] = item["cache_control"]
+                        content_list.append(content_block)
             return content_list
         else:
             raise ValueError(f"Invalid content type: {type(content)}")