Invalidate cache when --enable-incomplete-feature changes (fixes #20800)

Author: Kaushalt2004

mypy/options.py

@@ -76,6 +76,7 @@ class BuildType:
         "strict_bytes",
         "fixed_format_cache",
         "untyped_calls_exclude",
+        "enable_incomplete_feature",  # Added to ensure cache is invalidated when this flag changes
     }
 ) - {"debug_cache"}