fix(gemini): skip empty text parts in thinking blocks extraction

Gemini models were returning a trailing '{"text": ""}' in reasoning_content
because _extract_thinking_blocks_from_parts was including parts with empty
text content. These empty text parts with thoughtSignature were being
JSON-stringified and appended to reasoning_content in the streaming path.

Author: claude

litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py

@@ -1087,6 +1087,10 @@ def _extract_thinking_blocks_from_parts(
         thinking_blocks: List[ChatCompletionThinkingBlock] = []
         for part in parts:
             if "thoughtSignature" in part:
+                # Skip parts with empty text content to avoid adding '{"text": ""}' to reasoning_content
+                text_content = part.get("text", "")
+                if not text_content:
+                    continue
                 part_copy = part.copy()
                 part_copy.pop("thoughtSignature")
                 thinking_blocks.append(