refactor(tools): improve type hinting for credentials

Uses TYPE_CHECKING import to reference toolset_adk.CredentialConfig for the credentials parameter, improving static analysis without adding runtime dependencies.

Author: anubhav756

src/google/adk/tools/toolbox_toolset.py

@@ -19,6 +19,7 @@
 from typing import List
 from typing import Mapping
 from typing import Optional
+from typing import TYPE_CHECKING
 from typing import Union
 
 from typing_extensions import override
@@ -27,6 +28,9 @@
 from .base_tool import BaseTool
 from .base_toolset import BaseToolset
 
+if TYPE_CHECKING:
+  from toolbox_adk import CredentialConfig
+
 
 class ToolboxToolset(BaseToolset):
   """A class that provides access to toolbox toolsets.
@@ -55,7 +59,7 @@ def __init__(
       bound_params: Optional[
           Mapping[str, Union[Callable[[], Any], Any]]
       ] = None,
-      credentials: Optional[Any] = None,
+      credentials: Optional[CredentialConfig] = None,
       additional_headers: Optional[Mapping[str, str]] = None,
       **kwargs,
   ):