refactor(tools): Update ToolboxToolset to wrap toolbox-adk

### Description of Change

**Problem:**
The `ToolboxToolset` was implemented directly within `adk-python`, leading to code duplication and potential drift from the core `toolbox-adk` implementation.

**Solution:**
Refactored `ToolboxToolset` to act as a rigorous wrapper around the `toolbox-adk` package, which delegates all functionality to `toolbox_adk.ToolboxToolset`.

### Testing Plan

**Unit Tests:**

- [x] I have added or updated unit tests for my change.
- [x] All unit tests pass locally.

Summary:
-   Verified initialization flows through to `toolbox-adk`.
-   Verified `auth_token_getters` are correctly propagated.
-   Verified type hints are static-analysis friendly.

**Manual End-to-End (E2E) Tests:**

Manually verified standard toolbox loading and execution with the new wrapper:
```python
from google.adk.tools import ToolboxToolset
from toolbox_adk import CredentialStrategy

# Loading with toolset_name
ts = ToolboxToolset(
    server_url='http://localhost:8080',
    toolset_name='calculator',
    credentials=CredentialStrategy.toolbox_identity()
)
tools = await ts.get_tools()
print(f'Loaded {len(tools)} tools')
```

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.

PiperOrigin-RevId: 855798474

pyproject.toml

@@ -157,7 +157,7 @@ extensions = [
   "llama-index-readers-file>=0.4.0",            # For retrieval using LlamaIndex.
   "llama-index-embeddings-google-genai>=0.3.0", # For files retrieval using LlamaIndex.
   "lxml>=5.3.0",                                # For load_web_page tool.
-  "toolbox-core>=0.1.0",                        # For tools.toolbox_toolset.ToolboxToolset
+  "toolbox-adk>=0.1.0",                         # For tools.toolbox_toolset.ToolboxToolset
 ]
 
 otel-gcp = ["opentelemetry-instrumentation-google-genai>=0.3b0, <1.0.0"]

src/google/adk/tools/toolbox_toolset.py

@@ -12,29 +12,41 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 from typing import Any
 from typing import Callable
 from typing import List
 from typing import Mapping
 from typing import Optional
+from typing import TYPE_CHECKING
 from typing import Union
 
-import toolbox_core as toolbox
 from typing_extensions import override
 
 from ..agents.readonly_context import ReadonlyContext
 from .base_tool import BaseTool
 from .base_toolset import BaseToolset
-from .function_tool import FunctionTool
+
+if TYPE_CHECKING:
+  from toolbox_adk import CredentialConfig
 
 
 class ToolboxToolset(BaseToolset):
   """A class that provides access to toolbox toolsets.
 
+  This class acts as a bridge to the `toolbox-adk` package.
+  You must install `toolbox-adk` to use this class.
+
   Example:
   ```python
-  toolbox_toolset = ToolboxToolset("http://127.0.0.1:5000",
-  toolset_name="my-toolset")
+  from toolbox_adk import CredentialStrategy
+
+  toolbox_toolset = ToolboxToolset(
+      server_url="http://127.0.0.1:5000",
+      # toolset_name and tool_names are optional. If omitted, all tools are
+      loaded.
+      credentials=CredentialStrategy.toolbox_identity()
   )
   ```
   """
@@ -44,64 +56,58 @@ class ToolboxToolset(BaseToolset):
       server_url: str,
       toolset_name: Optional[str] = None,
       tool_names: Optional[List[str]] = None,
-      auth_token_getters: Optional[dict[str, Callable[[], str]]] = None,
+      auth_token_getters: Optional[Mapping[str, Callable[[], str]]] = None,
       bound_params: Optional[
           Mapping[str, Union[Callable[[], Any], Any]]
       ] = None,
+      credentials: Optional[CredentialConfig] = None,
+      additional_headers: Optional[Mapping[str, str]] = None,
+      **kwargs,
   ):
     """Args:
 
-      server_url: The URL of the toolbox server.
-      toolset_name: The name of the toolbox toolset to load.
-      tool_names: The names of the tools to load.
-      auth_token_getters: A mapping of authentication service names to
-        callables that return the corresponding authentication token. see:
-        https://github.com/googleapis/mcp-toolbox-sdk-python/tree/main/packages/toolbox-core#authenticating-tools
-        for details.
-      bound_params: A mapping of parameter names to bind to specific values or
-        callables that are called to produce values as needed. see:
-        https://github.com/googleapis/mcp-toolbox-sdk-python/tree/main/packages/toolbox-core#binding-parameter-values
-        for details.
-    The resulting ToolboxToolset will contain both tools loaded by tool_names
-    and toolset_name.
+    server_url: The URL of the toolbox server.
+    toolset_name: The name of the toolbox toolset to load.
+    tool_names: The names of the tools to load.
+    auth_token_getters: (Deprecated) Map of auth token getters.
+    bound_params: Parameters to bind to the tools.
+    credentials: (Optional) toolbox_adk.CredentialConfig object.
+    additional_headers: (Optional) Static headers dictionary.
+    **kwargs: Additional arguments passed to the underlying
+    toolbox_adk.ToolboxToolset.
     """
-    if not tool_names and not toolset_name:
-      raise ValueError("tool_names and toolset_name cannot both be None")
+    if not toolset_name and not tool_names:
+      raise ValueError(
+          "Either 'toolset_name' or 'tool_names' must be provided."
+      )
+
+    try:
+      from toolbox_adk import ToolboxToolset as RealToolboxToolset  # pylint: disable=import-outside-toplevel
+    except ImportError as exc:
+      raise ImportError(
+          "ToolboxToolset requires the 'toolbox-adk' package. "
+          "Please install it using `pip install toolbox-adk`."
+      ) from exc
+
     super().__init__()
-    self._server_url = server_url
-    self._toolbox_client = toolbox.ToolboxClient(server_url)
-    self._toolset_name = toolset_name
-    self._tool_names = tool_names
-    self._auth_token_getters = auth_token_getters or {}
-    self._bound_params = bound_params or {}
+
+    self._delegate = RealToolboxToolset(
+        server_url=server_url,
+        toolset_name=toolset_name,
+        tool_names=tool_names,
+        credentials=credentials,
+        additional_headers=additional_headers,
+        bound_params=bound_params,
+        auth_token_getters=auth_token_getters,
+        **kwargs,
+    )
 
   @override
   async def get_tools(
       self, readonly_context: Optional[ReadonlyContext] = None
   ) -> list[BaseTool]:
-    tools = []
-    if self._toolset_name:
-      tools.extend([
-          FunctionTool(tool)
-          for tool in await self._toolbox_client.load_toolset(
-              self._toolset_name,
-              auth_token_getters=self._auth_token_getters,
-              bound_params=self._bound_params,
-          )
-      ])
-    if self._tool_names:
-      tools.extend([
-          FunctionTool(
-              await self._toolbox_client.load_tool(
-                  tool_name,
-                  auth_token_getters=self._auth_token_getters,
-                  bound_params=self._bound_params,
-              )
-          )
-          for tool_name in self._tool_names
-      ])
-    return tools
+    return await self._delegate.get_tools(readonly_context)
 
   @override
   async def close(self):
-    self._toolbox_client.close()
+    await self._delegate.close()