fix: Remove thoughts from contents in llm requests

Merge #2320

Fix #843

COPYBARA_INTEGRATE_REVIEW=#2320 from CAROLZXYZXY:cazheng/fix-843 5b4a4b2
PiperOrigin-RevId: 790592793

Author: CAROLZXYZXY

src/google/adk/flows/llm_flows/contents.py

@@ -322,7 +322,8 @@ def _convert_foreign_event(event: Event) -> Event:
   content.role = 'user'
   content.parts = [types.Part(text='For context:')]
   for part in event.content.parts:
-    if part.text:
+    # Exclude thoughts from the context.
+    if part.text and not part.thought:
       content.parts.append(
           types.Part(text=f'[{event.author}] said: {part.text}')
       )