Add set_extractors() to TemporalBufferManager for buffer reconfiguration

Adds public API for replacing all extractors on a buffer, enabling
reconfiguration when subscribers are removed. This allows buffers to
optimize from TemporalBuffer to SingleValueBuffer when temporal
extractors are no longer needed.

Implementation:
- Add set_extractors() method that replaces entire extractor list
- Extract common buffer reconfiguration logic to _reconfigure_buffer_if_needed()
- Refactor add_extractor() to use the helper method

Test changes:
- Rename test_add_extractor_switches_to_single_value_buffer to
  test_set_extractors_switches_to_single_value_buffer
- Remove private field access (state.extractors.clear()) in test
- Use new public set_extractors() API instead

This prepares for DataService.unregister_subscriber() implementation,
which will need to recalculate extractors when subscribers are removed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: SimonHeybrock

src/ess/livedata/dashboard/temporal_buffer_manager.py

@@ -131,7 +131,43 @@ def add_extractor(self, key: K, extractor: UpdateExtractor) -> None:
         """
         state = self._states[key]
         state.extractors.append(extractor)
+        self._reconfigure_buffer_if_needed(key, state)
 
+    def set_extractors(self, key: K, extractors: list[UpdateExtractor]) -> None:
+        """
+        Replace all extractors for an existing buffer.
+
+        May trigger buffer type switch with data migration:
+        - Single→Temporal: Existing data is copied to the new buffer
+        - Temporal→Single: Last time slice is copied to the new buffer
+        - Other transitions: Data is discarded
+
+        Useful for reconfiguring buffers when subscribers change, e.g., when
+        a subscriber is removed and we need to downgrade from Temporal to
+        SingleValue buffer.
+
+        Parameters
+        ----------
+        key:
+            Key identifying the buffer to update.
+        extractors:
+            New list of extractors that will use this buffer.
+        """
+        state = self._states[key]
+        state.extractors = list(extractors)
+        self._reconfigure_buffer_if_needed(key, state)
+
+    def _reconfigure_buffer_if_needed(self, key: K, state: _BufferState) -> None:
+        """
+        Check if buffer type needs to change and handle migration.
+
+        Parameters
+        ----------
+        key:
+            Key identifying the buffer.
+        state:
+            Buffer state to reconfigure.
+        """
         # Check if we need to switch buffer type
         new_buffer = self._create_buffer_for_extractors(state.extractors)
         if not isinstance(new_buffer, type(state.buffer)):

tests/dashboard/temporal_buffer_manager_test.py

@@ -128,7 +128,7 @@ def test_add_extractor_switches_to_temporal_buffer(self):
         # Verify the data values are preserved
         assert sc.allclose(result['time', 0].data, data.data)
 
-    def test_add_extractor_switches_to_single_value_buffer(self):
+    def test_set_extractors_switches_to_single_value_buffer(self):
         """Test that switching buffer types preserves latest data."""
         manager = TemporalBufferManager()
         extractors = [WindowAggregatingExtractor(window_duration_seconds=1.0)]
@@ -152,12 +152,8 @@ def test_add_extractor_switches_to_single_value_buffer(self):
         assert 'time' in result.dims
         assert result.sizes['time'] == 3
 
-        # Manually clear extractors to simulate reconfiguration
-        state = manager._states['test']
-        state.extractors.clear()
-
-        # Add LatestValueExtractor - this should trigger buffer type switch
-        manager.add_extractor('test', LatestValueExtractor())
+        # Replace extractors - this should trigger buffer type switch
+        manager.set_extractors('test', [LatestValueExtractor()])
 
         # Verify the latest time slice is preserved after transition
         result = manager.get_buffered_data('test')