Introduce a unified Virtual Threads friendly API

The MCP Java SDK team and myself would like to acknowledge the strong desire to evolve the MCP Java SDK to leverage Virtual Threads as a key building block.

This issue is about introducing a new unified Virtual Threads friendly API with the following characteristics:

• Natively designed for Virtual Threads both in terms of idiomatic API and implementation
• Designed with Structured Concurrency in mind but SC will not be required
• Introduce a new way to expose streaming based features
• Extensible design that should allow changing the implementation
• No dependency on Reactor or Reactive Streams
• No Sync or Async prefix
• Technically could run on Java 21+ but Java 25+ (latest LTS version without the thread pinning issue) will be the recommended baseline

The Sync and Async API and implementation would remain close to what they are today for now, to not break existing use cases and remain fully compatible with Java 17+, while the new unified API would potentially mean a changed module hierarchy where Project Reactor is not a required dependency.

A prototype of this unified Virtual Threads friendly API, using the Sync API as the starting point but achieving the characteristics shared above, will be shared in this issue or via a related PR and discussed with the community to gather feedback and hopefully reach a consensus.

首先审查现有的 Sync 和 Async API 以及当前的模块层次结构，然后将它们对 Reactor 和 Reactive Streams 的依赖与提议的 Java 21+/25+ 方向进行比较。该 issue 没有提供可运行的文件或测试；进展意味着产出并讨论一个满足所列 Virtual Threads、流式处理、可扩展性和依赖项要求，并达成社区共识的原型。