fix(sdk-py): use correct f-string representation when loading error (#6388)

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## Description
- Fixed a bug in `libs/sdk-py/langgraph_sdk/auth/__init__.py` where the
error message for an already-set authentication handler did not properly
render the handler value.
- Updated the error string from a static `{self._authenticate_handler}`
to a correctly interpolated f-string.

```python
"Authentication handler already set as {self._authenticate_handler}."
```

```python
f"Authentication handler already set as {self._authenticate_handler}."
```

- Error messages now correctly display the actual handler instance,
improving debugging clarity.



- **Issue:** Fixes #6387
  - **Dependencies:** -
  - **Twitter handle:** -

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Author: Kavya-24

libs/sdk-py/langgraph_sdk/auth/__init__.py

@@ -259,7 +259,7 @@ async def authenticate(
         """
         if self._authenticate_handler is not None:
             raise ValueError(
-                "Authentication handler already set as {self._authenticate_handler}."
+                f"Authentication handler already set as {self._authenticate_handler}."
             )
         self._authenticate_handler = fn
         return fn