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.

mcp-core/src/main/java/io/modelcontextprotocol/util/Assert.java から始め、リポジトリ内で reactor.util.annotation.Nullable と関連する Reactor の null アノテーションを検索します。ビルド設定で JSpecify 依存関係を確認し、その後、関連するすべてのアノテーションが JSpecify を使用していること、また javax アノテーションクラスを必要とせずに SDK をビルドできることを検証します。