fix: narrow Agent.as_tool return type to FunctionTool

[seratch]

This pull request fixes a typing mismatch where Agent.as_tool() was annotated as returning Tool even though it always constructs and returns a FunctionTool.

• It updates agent.py to declare as_tool(...) -> FunctionTool.
• It removes now-redundant cast(FunctionTool, ...) patterns in tests
• It does not change runtime behavior; this change improves public type accuracy and prepares for future FunctionTool-specific extensions.