chore: Add type assertions and ignore type overlap in genai libraries

PiperOrigin-RevId: 869769923

Author: Annhiluc

google/genai/_interactions/_utils/_compat.py

@@ -42,7 +42,7 @@ def is_union(tp: Optional[Type[Any]]) -> bool:
     else:
         import types
 
-        return tp is Union or tp is types.UnionType
+        return tp is Union or tp is types.UnionType  # type: ignore[comparison-overlap]
 
 
 def is_typeddict(tp: Type[Any]) -> bool: