fix: make renewal feature opt-in (max_generations defaults to None)

Change max_generations from int=999 to int|None=None so the renewal
feature is fully disabled by default rather than hidden behind an
arbitrary high number.

Author: marklubin

python/semantic_kernel/contents/history_reducer/chat_history_double_buffer_reducer.py

@@ -98,7 +98,7 @@ class ChatHistoryDoubleBufferReducer(ChatHistoryReducer):
         swap_threshold: Fraction of target_count at which to swap buffers
             (0.0-1.0). Default 0.95. Must be greater than checkpoint_threshold.
         max_generations: Maximum number of summary-on-summary layers before
-            triggering renewal. Default 5.
+            triggering renewal. None means no limit (renewal disabled).
         renewal_policy: How to handle accumulated compression debt when
             max_generations is reached. Default RECURSE.
         summarization_instructions: The summarization prompt template.
@@ -121,11 +121,10 @@ class ChatHistoryDoubleBufferReducer(ChatHistoryReducer):
         le=1.0,
         description="Fraction of target_count at which to swap to the back buffer.",
     )
-    max_generations: int = Field(
-        default=999,
-        gt=0,
+    max_generations: int | None = Field(
+        default=None,
         description="Maximum summary-on-summary layers before renewal. "
-        "Set to a lower value to enable incremental summary accumulation.",
+        "None means no limit (renewal disabled).",
     )
     renewal_policy: RenewalPolicy = Field(
         default=RenewalPolicy.RECURSE,
@@ -263,7 +262,7 @@ async def _create_checkpoint(self) -> Self | None:
 
         try:
             # Check if we need renewal first
-            if self._current_generation >= self.max_generations:
+            if self.max_generations is not None and self._current_generation >= self.max_generations:
                 await self._perform_renewal()
                 # Re-capture — renewal may have reassigned self.messages
                 history = self.messages

python/tests/unit/contents/test_chat_history_double_buffer_reducer.py

@@ -77,7 +77,7 @@ def test_double_buffer_reducer_defaults(mock_service):
     reducer = ChatHistoryDoubleBufferReducer(service=mock_service, target_count=10)
     assert reducer.checkpoint_threshold == 0.7
     assert reducer.swap_threshold == 0.95
-    assert reducer.max_generations == 999
+    assert reducer.max_generations is None
     assert reducer.renewal_policy == RenewalPolicy.RECURSE
     assert reducer.fail_on_error is True
     assert reducer.generation == 0