Match return type of SerializationInfo.mode() to mode of model_dump()

Pydantic sometimes uses Literal['python', 'json'] | str for the
serialization mode. Using a more restrictive type can cause issues
(e.g., typing validation errors).

This commit adjusts the return type to align with other valid
Pydantic serialization modes and prevent such issues.

This can be seen as a follow-up to commit:
9b239c1

Author: Julian Vanden Broeck

python/pydantic_core/core_schema.py

@@ -145,7 +145,7 @@ def context(self) -> ContextT:
         ...
 
     @property
-    def mode(self) -> Literal['python', 'json']:
+    def mode(self) -> Literal['python', 'json'] | str:
         """The serialization mode set during serialization."""
         ...