[Lens as code] Fix waffle chart legend values not roundtripping through the API (elastic#269774)

## Summary

fix elastic#268819

The waffle chart is the only partition chart that lets users show/hide
metric values in the legend. However, the API transform layer was
missing the mapping between the API legend.values field and the internal
legendStats state — meaning this setting was silently lost when
converting to/from the API format.

This adds bidirectional mapping:

- `legend.values: ['absolute']` → `legendStats: ['value']` (show values)
- `legend.values` omitted → `legendStats: []` (hide values)

For existing saved objects where `legendStats` is `undefined` (which
renders as "show values" by default), serializing to API now correctly
emits `legend.values: ['absolute']`.

Author: markov00

src/platform/packages/shared/kbn-lens-embeddable-utils/config_builder/tests/partition/partition.test.ts

@@ -203,4 +203,77 @@ describe('Partition', () => {
       expect(apiOutput.metrics[0].color).toEqual(AUTO_COLOR);
     });
   });
+
+  describe('waffle legend values', () => {
+    const baseDataSource = {
+      type: AS_CODE_DATA_VIEW_SPEC_TYPE,
+      index_pattern: 'test-index',
+      time_field: '@timestamp',
+    } as const;
+
+    const baseWaffleConfig = {
+      type: 'waffle',
+      title: 'Waffle legend test',
+      data_source: baseDataSource,
+      metrics: [{ operation: 'count', empty_as_null: false }],
+      group_by: [
+        {
+          operation: 'terms',
+          fields: ['tags.keyword'],
+          limit: 3,
+        },
+      ],
+      sampling: 1,
+      ignore_global_filters: false,
+    } satisfies WaffleConfig;
+
+    it('should map legend.values: ["absolute"] to legendStats: ["value"] in state', () => {
+      const config = {
+        ...baseWaffleConfig,
+        legend: { values: ['absolute'] as ['absolute'] },
+      } satisfies WaffleConfig;
+
+      const builder = new LensConfigBuilder();
+      const lensState = builder.fromAPIFormat(config);
+      const vizState = lensState.state.visualization as LensPartitionVisualizationState;
+
+      expect(vizState.layers[0].legendStats).toEqual(['value']);
+    });
+
+    it('should map omitted legend.values to legendStats: [] in state', () => {
+      const builder = new LensConfigBuilder();
+      const lensState = builder.fromAPIFormat(baseWaffleConfig);
+      const vizState = lensState.state.visualization as LensPartitionVisualizationState;
+
+      expect(vizState.layers[0].legendStats).toEqual([]);
+    });
+
+    it('should roundtrip legend.values: ["absolute"] correctly', () => {
+      const config = {
+        ...baseWaffleConfig,
+        legend: { values: ['absolute'] as ['absolute'] },
+      } satisfies WaffleConfig;
+
+      const builder = new LensConfigBuilder();
+      const lensState = builder.fromAPIFormat(config);
+      const apiOutput = builder.toAPIFormat(lensState) as WaffleConfig;
+
+      expect(apiOutput.legend?.values).toEqual(['absolute']);
+    });
+
+    it('should roundtrip omitted legend.values correctly', () => {
+      const builder = new LensConfigBuilder();
+      const lensState = builder.fromAPIFormat(baseWaffleConfig);
+      const apiOutput = builder.toAPIFormat(lensState) as WaffleConfig;
+
+      expect(apiOutput.legend?.values).toBeUndefined();
+    });
+
+    it('should serialize existing waffle state without legendStats as legend.values: ["absolute"]', () => {
+      const builder = new LensConfigBuilder(undefined, true);
+      const apiOutput = builder.toAPIFormat(waffleLegacyBasicState) as WaffleConfig;
+
+      expect(apiOutput.legend?.values).toEqual(['absolute']);
+    });
+  });
 });

src/platform/packages/shared/kbn-lens-embeddable-utils/config_builder/transforms/charts/partition.ts

@@ -232,6 +232,34 @@ function convertAPILegendDisplayOption(
   return { legendDisplay: legend?.visibility === 'visible' ? 'show' : 'hide', ...legendOptions };
 }
 
+/**
+ * Waffle is the only partition chart that supports showing/hiding metric values in the legend.
+ * Internally, Lens uses `legendStats` (empty array = hidden, ['value'] = shown).
+ * The API schema represents this as `legend.values: ['absolute']` (shown) or omitted (hidden).
+ */
+function convertWaffleLegendValuesToState(
+  values: NonNullable<WaffleConfig['legend']>['values']
+): LensPartitionLayerState['legendStats'] {
+  if (values?.includes('absolute')) {
+    return ['value'];
+  }
+  return [];
+}
+
+/**
+ * Converts internal `legendStats` back to the API `legend.values` field for waffle charts.
+ * When `legendStats` is undefined (legacy saved objects), values are shown by default
+ * (matching the rendering behavior in `getLegendStats`), so we emit `['absolute']`.
+ */
+function convertLegendStatsToWaffleAPIValues(
+  legendStats: LensPartitionLayerState['legendStats']
+): NonNullable<WaffleConfig['legend']>['values'] {
+  if (legendStats === undefined || legendStats.includes('value')) {
+    return ['absolute'];
+  }
+  return undefined;
+}
+
 function convertAPIStaticColorToLensState(config: PartitionConfig) {
   if (isAPIMosaicChartLayer(config)) {
     return undefined;
@@ -381,6 +409,7 @@ function buildVisualizationState(
           ...sharedState,
           ...(!isLegacyColor && { ...colorMapping }),
           categoryDisplay: 'default',
+          legendStats: convertWaffleLegendValuesToState(config.legend?.values),
         },
       ],
     };
@@ -468,6 +497,9 @@ function fromLensStateToSharedPartitionAPI(
     truncate_after_lines: getLegendTruncateAfterLines(layerState),
     nested: isStateWaffleChart(visualization) ? undefined : layerState.nestedLegend,
     size: legendSizeCompat.toAPI(layerState.legendSize),
+    values: isStateWaffleChart(visualization)
+      ? convertLegendStatsToWaffleAPIValues(layerState.legendStats)
+      : undefined,
   });
 
   return stripUndefined({