fix(dashboard): offer Exit edit mode when there is nothing to discard (#42208)

Co-authored-by: Claude Code <noreply@anthropic.com>

superset-frontend/src/dashboard/components/Header/Header.test.tsx

diff --git a/superset-frontend/src/dashboard/components/Header/Header.test.tsx b/superset-frontend/src/dashboard/components/Header/Header.test.tsx
index 98397ec..6a6e5e4 100644
--- a/superset-frontend/src/dashboard/components/Header/Header.test.tsx
+++ b/superset-frontend/src/dashboard/components/Header/Header.test.tsx
@@ -522,12 +522,15 @@
   expect(onRedo).not.toHaveBeenCalled();
 });
 
-test('should render the "Discard" button as disabled', () => {
+test('should render an enabled "Exit edit mode" button when there are no unsaved changes', () => {
   setup(editableState);
-  expect(screen.getByRole('button', { name: /discard/i })).toBeDisabled();
+  expect(screen.getByRole('button', { name: /exit edit mode/i })).toBeEnabled();
+  expect(
+    screen.queryByRole('button', { name: /discard/i }),
+  ).not.toBeInTheDocument();
 });
 
-test('should enable the "Discard" button when there are unsaved changes', () => {
+test('should render an enabled "Discard" button when there are unsaved changes', () => {
   const unsavedState = {
     ...editableState,
     dashboardState: {
@@ -537,6 +540,9 @@
   };
   setup(unsavedState);
   expect(screen.getByRole('button', { name: /discard/i })).toBeEnabled();
+  expect(
+    screen.queryByRole('button', { name: /exit edit mode/i }),
+  ).not.toBeInTheDocument();
 });
 
 test('should render the "Save" button as disabled', () => {

superset-frontend/src/dashboard/components/Header/index.tsx

diff --git a/superset-frontend/src/dashboard/components/Header/index.tsx b/superset-frontend/src/dashboard/components/Header/index.tsx
index ca3b875..283ff04 100644
--- a/superset-frontend/src/dashboard/components/Header/index.tsx
+++ b/superset-frontend/src/dashboard/components/Header/index.tsx
@@ -722,13 +722,14 @@
                 <Button
                   css={discardBtnStyle}
                   buttonSize="small"
-                  disabled={!hasUnsavedChanges}
                   onClick={discardChanges}
                   buttonStyle="secondary"
                   data-test="discard-changes-button"
-                  aria-label={t('Discard')}
+                  aria-label={
+                    hasUnsavedChanges ? t('Discard') : t('Exit edit mode')
+                  }
                 >
-                  {t('Discard')}
+                  {hasUnsavedChanges ? t('Discard') : t('Exit edit mode')}
                 </Button>
                 <Button
                   css={saveBtnStyle}