refactor: remove obsolete Flask flash messaging system

BREAKING CHANGE: Removes flask.flash() messaging infrastructure

The Flask flash message system is a legacy feature from when Superset was a
server-rendered application. In the current SPA architecture, these messages
are never displayed to users because:
- Flash messages only render on page load via FlashProvider's componentDidMount
- Modern UI interactions use async API calls that don't trigger page reloads
- The main consumer (/explore/ POST endpoint) is already marked @deprecated

All user-facing notifications are already handled by the frontend toast system,
including chart save operations (see saveModalActions.ts) which dispatches
success toasts for:
- Chart saved/overwritten
- Chart added to dashboard
- New dashboard created with chart

Changes:
- Backend: Removed all flash() calls from views (14 occurrences in 4 files)
- Backend: Converted error flashes to JSON responses or logging
- Backend: Removed redirect_with_flash utility function
- Frontend: Deleted FlashProvider component and tests
- Frontend: Removed flash_messages from CommonBootstrapData type
- Frontend: Cleaned up context providers and test fixtures
- Frontend: Removed unused getBootstrapData imports

The application now relies entirely on client-side toast notifications for
user feedback, which properly support async operations and provide a
consistent UX.

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

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

Author: mistercrunch

superset-frontend/spec/fixtures/mockDashboardInfo.js

@@ -49,7 +49,6 @@ export default {
   dash_edit_perm: true,
   dash_save_perm: true,
   common: {
-    flash_messages: [],
     conf: { SUPERSET_WEBSERVER_TIMEOUT: 60 },
   },
   filterBarOrientation: FilterBarOrientation.Vertical,

superset-frontend/src/components/FlashProvider/FlashProvider.test.tsx

@@ -38,7 +38,6 @@ export * from './ErrorMessage';
 export { ImportModal, type ImportModelsModalProps } from './ImportModal';
 export { ErrorBoundary, type ErrorBoundaryProps } from './ErrorBoundary';
 export * from './GenericLink';
-export { FlashProvider, type FlashMessage } from './FlashProvider';
 export { GridTable, type TableProps } from './GridTable';
 export * from './Tag';
 export * from './TagsList';

superset-frontend/src/components/FlashProvider/index.tsx

@@ -122,7 +122,6 @@ export const RESERVED_DASHBOARD_URL_PARAMS: string[] = [
 export const DEFAULT_COMMON_BOOTSTRAP_DATA: CommonBootstrapData = {
   application_root: '/',
   static_assets_prefix: '',
-  flash_messages: [],
   conf: {},
   locale: 'en',
   feature_flags: {},

superset-frontend/src/components/FlashProvider/types.ts

@@ -274,7 +274,6 @@ export const hydrateDashboard =
           superset_can_csv: findPermission('can_csv', 'Superset', roles),
           common: {
             // legacy, please use state.common instead
-            flash_messages: common?.flash_messages,
             conf: common?.conf,
           },
           filterBarOrientation:

superset-frontend/src/components/index.ts

@@ -22,13 +22,12 @@ import { Provider as ReduxProvider } from 'react-redux';
 import { QueryParamProvider } from 'use-query-params';
 import { DndProvider } from 'react-dnd';
 import { HTML5Backend } from 'react-dnd-html5-backend';
-import { FlashProvider, DynamicPluginProvider } from 'src/components';
+import { DynamicPluginProvider } from 'src/components';
 import { EmbeddedUiConfigProvider } from 'src/components/UiConfigContext';
 import { SupersetThemeProvider } from 'src/theme/ThemeProvider';
 import { ThemeController } from 'src/theme/ThemeController';
 import type { ThemeStorage } from '@superset-ui/core';
 import { store } from 'src/views/store';
-import getBootstrapData from 'src/utils/getBootstrapData';
 
 /**
  * In-memory implementation of ThemeStorage interface for embedded contexts.
@@ -56,7 +55,6 @@ const themeController = new ThemeController({
 
 export const getThemeController = (): ThemeController => themeController;
 
-const { common } = getBootstrapData();
 const extensionsRegistry = getExtensionsRegistry();
 
 export const EmbeddedContextProviders: React.FC = ({ children }) => {
@@ -68,24 +66,22 @@ export const EmbeddedContextProviders: React.FC = ({ children }) => {
     <SupersetThemeProvider themeController={themeController}>
       <ReduxProvider store={store}>
         <DndProvider backend={HTML5Backend}>
-          <FlashProvider messages={common.flash_messages}>
-            <EmbeddedUiConfigProvider>
-              <DynamicPluginProvider>
-                <QueryParamProvider
-                  ReactRouterRoute={Route}
-                  stringifyOptions={{ encode: false }}
-                >
-                  {RootContextProviderExtension ? (
-                    <RootContextProviderExtension>
-                      {children}
-                    </RootContextProviderExtension>
-                  ) : (
-                    children
-                  )}
-                </QueryParamProvider>
-              </DynamicPluginProvider>
-            </EmbeddedUiConfigProvider>
-          </FlashProvider>
+          <EmbeddedUiConfigProvider>
+            <DynamicPluginProvider>
+              <QueryParamProvider
+                ReactRouterRoute={Route}
+                stringifyOptions={{ encode: false }}
+              >
+                {RootContextProviderExtension ? (
+                  <RootContextProviderExtension>
+                    {children}
+                  </RootContextProviderExtension>
+                ) : (
+                  children
+                )}
+              </QueryParamProvider>
+            </DynamicPluginProvider>
+          </EmbeddedUiConfigProvider>
         </DndProvider>
       </ReduxProvider>
     </SupersetThemeProvider>

superset-frontend/src/constants.ts

@@ -98,7 +98,6 @@ export interface ExploreResponsePayload {
 export interface ExplorePageState {
   user: UserWithPermissionsAndRoles;
   common: {
-    flash_messages: string[];
     conf: JsonObject;
     locale: string;
   };

superset-frontend/src/dashboard/actions/hydrate.js

@@ -83,7 +83,6 @@ const createMockBootstrapData = (
   common: {
     application_root: '/',
     static_assets_prefix: '/static/assets/',
-    flash_messages: [],
     conf: {},
     locale: 'en',
     feature_flags: {},
@@ -391,7 +390,6 @@ describe('ThemeController', () => {
       common: {
         application_root: '/',
         static_assets_prefix: '/static/assets/',
-        flash_messages: [],
         conf: {},
         locale: 'en',
         feature_flags: {},