fix(adk): implement _get_declaration to natively support ADK schema builder

[anubhav756]

This PR implements the _get_declaration method in ToolboxTool for the toolbox-adk package. This enables the ADK tool wrapper to natively provide a FunctionDeclaration schema directly to ADK orchestration, correctly exposing all underlying tool parameters, their types, and descriptions.

Because it wraps a ToolboxTool from toolbox-adk wraps that from toolbox-core, the framework's default reflection mechanisms struggles to parse the expected parameter signature. By explicitly constructing the Schema and FunctionDeclaration objects using the definitions present on the core tool, we ensure complete compatibility with the ADK's native schema builder.