[v9.1.x] Grafana/schema: Fix plugins API regression. Add back "hidden" in LegendDisplayMode (#53966)

Co-authored-by: Esteban Beltran <academo@users.noreply.github.com>

packages/grafana-schema/src/schema/mudball.cue

@@ -133,7 +133,8 @@ GraphThresholdsStyleConfig: {
 LegendPlacement: "bottom" | "right" @cuetsy(kind="type")
 
 // TODO docs
-LegendDisplayMode: "list" | "table" @cuetsy(kind="enum")
+// Note: "hidden" needs to remain as an option for plugins compatibility
+LegendDisplayMode: "list" | "table" | "hidden" @cuetsy(kind="enum")
 
 // TODO docs
 TableSortByFieldState: {

packages/grafana-schema/src/schema/mudball.gen.ts

@@ -168,6 +168,7 @@ export interface GraphThresholdsStyleConfig {
 export type LegendPlacement = ('bottom' | 'right');
 
 export enum LegendDisplayMode {
+  Hidden = 'hidden',
   List = 'list',
   Table = 'table',
 }