fix(backend): increment preserved_count when class_name fallback succeeds

When block.name lookup fails but class_name fallback finds manual content,
preserved_count was not incremented, causing underreported statistics.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

autogpt_platform/backend/scripts/migrate_block_docs.py

@@ -172,7 +172,9 @@ def run_migration():
             else:
                 # Try with class name
                 manual_content = manual_content_cache.get(block.class_name, {})
-                if not manual_content:
+                if manual_content:
+                    preserved_count += 1
+                else:
                     missing_count += 1
 
             content_parts.append(