chore: drop legacy /superset prefix from remaining hardcoded URLs

Following #39925, which collapsed the historical `/superset/` prefix off
all canonical views, a few hardcoded literals still emit the old form and
rely on the closed-set 308 redirect shim (scheduled for removal at EOL
5.0.0). Point them at the canonical paths directly:

- deck.gl and legacy nvd3 `getURIDirectory()`:
  `/superset/explore_json/` -> `/explore_json/`
- filter-box -> native-filter migration markdown href:
  `/superset/slice/{id}/` -> `/slice/{id}/`

The migration-helper change only affects future upgrade runs; markdown
already written to migrated dashboards is untouched (and still shim-covered
until 5.0.0). No tests pin these literals, and `/explore_json/` and
`/slice/<int>/` are live routes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

superset-frontend/plugins/legacy-preset-chart-nvd3/src/vendor/superset/exploreUtils.ts

@@ -27,7 +27,7 @@ export function getURIDirectory(formData, endpointType = 'base') {
   // Building the directory part of the URI
   let directory = '/explore/';
   if (['json', 'csv', 'query', 'results', 'samples'].includes(endpointType)) {
-    directory = '/superset/explore_json/';
+    directory = '/explore_json/';
   }
 
   return directory;

superset-frontend/plugins/preset-chart-deckgl/src/utils/explore.ts

@@ -26,7 +26,7 @@ export function getURIDirectory(endpointType = 'base') {
   // Building the directory part of the URI
   let directory = '/explore/';
   if (['json', 'csv', 'query', 'results', 'samples'].includes(endpointType)) {
-    directory = '/superset/explore_json/';
+    directory = '/explore_json/';
   }
 
   return directory;

superset/migrations/shared/native_filters.py

@@ -335,7 +335,7 @@ def migrate_dashboard(dashboard: Dashboard) -> None:  # noqa: C901
 
                 meta["code"] = dedent(
                     f"""
-                        &#9888; The <a href="/superset/slice/{slc.id}/">{slc.slice_name}
+                        &#9888; The <a href="/slice/{slc.id}/">{slc.slice_name}
                         </a> filter-box chart has been migrated to a native filter.
                         """
                 )