Migrate null annotations to jspecify

Currently, the sdk uses Reactor's reactor.util.annotation.Nullable annotations, such as here

https://github.com/modelcontextprotocol/java-sdk/blob/fd004989b9484c9b81be6b03463396797b354804/mcp-core/src/main/java/io/modelcontextprotocol/util/Assert.java#L28

This is causing warnings in my downstream build since that type refers to javax classes I don't have on the classpath.

This is Reactor Nullable, along with its imports:

import javax.annotation.Nonnull;
import javax.annotation.meta.TypeQualifierNickname;
import javax.annotation.meta.When;

@Nonnull(when = When.MAYBE)
@TypeQualifierNickname
@Deprecated
public @interface Nullable {
}

The Reactor type is now deprecated, and they recommend you using JSpecify which is fast becoming the standard, if it is not already.

I can possibly submit a PR for this, but I notice a lot of PRs opened recently have limited activity, so will wait for the go-ahead first.

Bắt đầu với mcp-core/src/main/java/io/modelcontextprotocol/util/Assert.java và tìm kiếm trong repository reactor.util.annotation.Nullable cùng các annotation null liên quan của Reactor. Kiểm tra cấu hình build để tìm dependency JSpecify, sau đó xác minh rằng tất cả các annotation liên quan đều sử dụng JSpecify và SDK có thể được build mà không yêu cầu các lớp annotation javax.