feat: migrate ToolboxToolset to use toolbox-adk and align validation

### Description of Change

**Problem:**
The `ToolboxToolset` was relying on the legacy `toolbox-core` package. Users wanting to use the Toolbox features were forced to install the heavy `[extensions]` group, lacking a granular installation option. Additionally, `ToolboxToolset` had a validation check enforcing either `toolset_name` or `tool_names` to be present, preventing the default behavior of loading all tools (which `toolbox-adk` supports).

**Solution:**
* Refactored `ToolboxToolset` to delegate to `toolbox-adk`.
* Added a new `toolbox` optional dependency group in `pyproject.toml`.
  * Users can now run `pip install google-adk[toolbox]` to install only the necessary dependencies.
* Updated the `extensions` dependency group to replace `toolbox-core` with `toolbox-adk`.
  * This ensures existing users of `[extensions]` are not broken upon upgrade.
* Removed the restrictive validation check to allow default loading of all tools.
* Updated the `ImportError` message to guide users toward the new granular installation command.

### Testing Plan

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

**Manual End-to-End (E2E) Tests:**
- Verified that the sample agent runs correctly with `toolbox-adk` locally.
- Verified that `ToolboxToolset` can now be instantiated without arguments to load all 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: 857171811

contributing/samples/toolbox_agent/README.md

@@ -26,10 +26,10 @@ Install SQLite from [https://sqlite.org/](https://sqlite.org/)
 
 ### 3. Install Required Python Dependencies
 
-**Important**: The ADK's `ToolboxToolset` class requires the `toolbox-core` package, which is not automatically installed with the ADK. Install it using:
+**Important**: The ADK's `ToolboxToolset` class requires the `toolbox-adk` package, which is not automatically installed with the ADK. Install it using:
 
 ```bash
-pip install toolbox-core
+pip install google-adk[toolbox]
 ```
 
 ### 4. Create Database (Optional)

pyproject.toml

@@ -157,11 +157,12 @@ 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-adk>=0.1.0",                         # For tools.toolbox_toolset.ToolboxToolset
+  "toolbox-adk>=0.5.7, <0.6.0",                 # For tools.toolbox_toolset.ToolboxToolset
 ]
 
 otel-gcp = ["opentelemetry-instrumentation-google-genai>=0.3b0, <1.0.0"]
 
+toolbox = ["toolbox-adk>=0.5.7, <0.6.0"]
 
 [tool.pyink]
 # Format py files following Google style-guide

src/google/adk/tools/toolbox_toolset.py

@@ -35,19 +35,9 @@ if TYPE_CHECKING:
 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
-  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()
-  )
+  toolbox_toolset = ToolboxToolset("http://127.0.0.1:5000")
   ```
   """
 
@@ -64,29 +54,37 @@ class ToolboxToolset(BaseToolset):
       additional_headers: Optional[Mapping[str, str]] = None,
       **kwargs,
   ):
-    """Args:
+    """Initializes the ToolboxToolset.
 
-    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.
+    Args:
+      server_url: The URL of the toolbox server.
+      toolset_name: (Optional) The name of the toolbox toolset to load.
+      tool_names: (Optional) The names of the tools to load.
+      auth_token_getters: (Optional) 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: (Optional) 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.
+      credentials: (Optional) toolbox_adk.CredentialConfig object.
+      additional_headers: (Optional) Static headers mapping.
+      **kwargs: Additional arguments passed to the underlying
+        toolbox_adk.ToolboxToolset.
+
+    The resulting ToolboxToolset will contain both tools loaded by tool_names
+    and toolset_name.
+
+    Note: toolset_name and tool_names are optional.
+    If both are omitted, all tools are loaded.
     """
-    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`."
+          "Please install it using `pip install google-adk[toolbox]`."
       ) from exc
 
     super().__init__()
@@ -95,10 +93,10 @@ class ToolboxToolset(BaseToolset):
         server_url=server_url,
         toolset_name=toolset_name,
         tool_names=tool_names,
+        auth_token_getters=auth_token_getters,
+        bound_params=bound_params,
         credentials=credentials,
         additional_headers=additional_headers,
-        bound_params=bound_params,
-        auth_token_getters=auth_token_getters,
         **kwargs,
     )