chore: Update comments in Compaction to clarify timestamp-based ranges

The docstrings for `compaction_range` and `compacted_content` are updated to reflect that compaction is based on timestamp ranges rather than sequence IDs, and to use consistent terminology ("compacted" instead of "summarized").

PiperOrigin-RevId: 808770610

Author: hangfei

src/google/adk/events/event_actions.py

@@ -36,11 +36,14 @@ class EventCompaction(BaseModel):
   )
   """The pydantic model config."""
 
-  compaction_range: Optional[tuple[float, float]] = None
-  """The sequence ID range of the events that are summarized, in the form(start_sequence_id, end_sequence_id)`"""
+  start_timestamp: float
+  """The start timestamp of the compacted events, in seconds."""
+
+  end_timestamp: float
+  """The end timestamp of the compacted events, in seconds."""
 
   compacted_content: Content
-  """The summarized content of the events."""
+  """The compacted content of the events."""
 
 
 class EventActions(BaseModel):