feat: make AgentExecutor a functional interface with default cancel m…

[malladinagarjuna2]

Overview

This PR converts the AgentExecutor interface into a @FunctionalInterface by providing a default implementation for the cancel method, allowing developers to define simple agents using lambda expressions.

Fixes #1009

The Problem

Previously, AgentExecutor required developers to implement two methods: execute and cancel. Because there were two abstract methods, it was impossible to use a lambda expression (e.g. (context, emitter) -> { ... }) to implement an agent. This forced developers to write verbose boilerplate even for simple fire-and-forget agents that do not require cancellation logic.

The Solution

1. Added @FunctionalInterface: Explicitly marked AgentExecutor as a functional interface.
2. Default cancel Implementation: Converted the cancel method to a default method. If an agent developer does not override it, it now correctly throws a TaskNotCancelableError("Agent execution is not cancelable.") by default, conforming to the existing A2A error specification.