fix(database): retry blur validation after a transient request failure

When the validate_parameters request failed without a structured error
body (e.g. network drop), getValidation returned an empty object that
the caller could not distinguish from "validation passed" — and the
blur dedup cache was already updated, so the same form state would never
revalidate until the user changed a field.

Have getValidation return null on unexpected failure and only update
the snapshot cache after a usable response.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

superset-frontend/src/features/databases/DatabaseModal/index.tsx

@@ -823,13 +823,19 @@ const DatabaseModal: FunctionComponent<DatabaseModalProps> = ({
     [onChange, handleClearValidationErrors],
   );
 
-  const getBlurValidation = useCallback(() => {
+  const getBlurValidation = useCallback(async () => {
     const currentDbSnapshot = JSON.stringify(db);
     if (currentDbSnapshot === lastValidatedDbSnapshotRef.current) {
-      return Promise.resolve([]);
+      return [];
     }
-    lastValidatedDbSnapshotRef.current = currentDbSnapshot;
-    return getValidation(db);
+    const result = await getValidation(db);
+    // Only cache after a request that produced a usable response. ``null``
+    // signals an unexpected/network failure, in which case we leave the
+    // snapshot untouched so the next blur retries.
+    if (result !== null) {
+      lastValidatedDbSnapshotRef.current = currentDbSnapshot;
+    }
+    return result;
   }, [db, getValidation]);
 
   const onClose = () => {

superset-frontend/src/views/CRUD/hooks.ts

@@ -909,7 +909,7 @@ export function useDatabaseValidation() {
           setIsValidating(false);
           setHasValidated(true);
         }
-        return {};
+        return null;
       }
     },
     [setValidationErrors],