fix: explicitly preserve language and tools during event compaction

Merge #6329

Fixes #6273

PiperOrigin-RevId: 944746917

Author: eajajhossain

src/google/adk/apps/llm_event_summarizer.py

@@ -50,7 +50,13 @@ class LlmEventSummarizer(BaseEventsSummarizer):
       ' It may or may not start from a compacted history. Please identify and'
       ' reiterate the user request, summarize the context so far, focusing on'
       ' key decisions made and information obtained, as well as any unresolved'
-      ' questions or tasks. The summary should be concise and capture the'
+      ' questions or tasks. '
+      'CRITICAL INSTRUCTIONS: '
+      '1. Explicitly identify and state the primary language used by the user '
+      'at the top of your summary (e.g., "Conversation Language: English"). '
+      '2. If the agent called any tools, accurately list the exact tool names '
+      'used to maintain tool grounding. '
+      'The rest of the summary should be concise and capture the'
       ' essence of the interaction.\n\n{conversation_history}'
   )