fix: LiteLLM Claude thinking blocks with display: "omitted" were lost

Starting with Opus 4.7 thoughts display defaults to omitted instead of summarized https://platform.claude.com/docs/en/build-with-claude/extended-thinking#controlling-thinking-display which omits thinking_text. However thinking signature is still critically important to be preserved and sent back to the model.

PiperOrigin-RevId: 934866189

Author: google-genai-bot

src/google/adk/models/lite_llm.py

@@ -525,7 +525,7 @@ def _convert_reasoning_value_to_parts(reasoning_value: Any) -> List[types.Part]:
         continue
       thinking_text = block.get("thinking", "")
       signature = block.get("signature", "")
-      if not thinking_text:
+      if not thinking_text and not signature:
         continue
       part = types.Part(text=thinking_text, thought=True)
       if signature: