Invalidate cache when --enable-incomplete-feature changes (fixes #20845) (#20849)

Fixes #20845

This PR ensures that the mypy cache is invalidated when the
--enable-incomplete-feature flag changes. Previously, enabling or
disabling incomplete features did not affect the cache key, so users
could get stale results unless they manually cleared the cache. Now, the
enable_incomplete_feature option is included in the cache key logic, so
the cache is properly refreshed when this flag changes.

---------

Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>

Author: Kaushalt2004

mypy/options.py

@@ -76,6 +76,7 @@ class BuildType:
         "strict_bytes",
         "fixed_format_cache",
         "untyped_calls_exclude",
+        "enable_incomplete_feature",
     }
 ) - {"debug_cache"}