Revert "Match return type of SerializationInfo.mode() to mode of model_dump() (#1770)"

This reverts commit d0dbb09.
See #1747 (comment)
for more details.

Author: Viicos

python/pydantic_core/core_schema.py

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