fix: Make compactor optional in EventsCompactionConfig and add a default

If `EventsCompactionConfig` is provided without a `compactor`, a `SlidingWindowCompactor` is now automatically instantiated using the `root_agent`'s LLM. This simplifies configuration by providing a sensible default.

PiperOrigin-RevId: 816038579

src/google/adk/apps/base_events_summarizer.py

@@ -0,0 +1,47 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from __future__ import annotations
+
+import abc
+from typing import Optional
+
+from google.genai.types import Content
+
+from ..events.event import Event
+from ..utils.feature_decorator import experimental
+
+
+@experimental
+class BaseEventsSummarizer(abc.ABC):
+  """Base interface for compacting events."""
+
+  @abc.abstractmethod
+  async def maybe_summarize_events(
+      self, *, events: list[Event]
+  ) -> Optional[Event]:
+    """Compact a list of events into a single event.
+
+    If compaction failed, return None. Otherwise, compact into a content and
+    return it.
+
+    This method will summarize the events and return a new summray event
+    indicating the range of events it summarized.
+
+    Args:
+      events: Events to compact.
+
+    Returns:
+      The new compacted event, or None if no compaction happended.
+    """
+    raise NotImplementedError()