### 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
Merge https://github.com/google/adk-python/pull/1195
## Summary
Updated the Toolbox Agent documentation to address a critical missing dependency that prevents the agent from running successfully.
## Changes Made
- **Added missing dependency**: Documented that `toolbox-core` must be installed via `pip install toolbox-core`
- **Improved documentation structure**: Added clear section numbering and better organization
- **Enhanced readability**: Fixed grammar, capitalization, and formatting throughout
- **Added Prerequisites section**: Set clear expectations before installation begins
- **Clarified optional steps**: Made it clearer when database creation can be skipped
## Problem Solved
The original documentation was missing a crucial step - installing the `toolbox-core` package. Without this dependency, users encounter an `ImportError: No module named 'toolbox-core'` when trying to use the `ToolboxToolset` class in ADK. This fix ensures users can successfully set up and run the agent without encountering import errors.
## Testing
- Verified the installation steps work correctly with the added dependency
- Confirmed the agent runs successfully after following the updated documentation
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/1195 from designcomputer:patch-1 b90c71fe95aa09a3dca069e91f14791f557ab2e3
PiperOrigin-RevId: 785487495
Google Team MemberandCopybara-Service
Dana K. WilliamsandCopybara-Service
Xiang (Sean) ZhouandCopybara-Service