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.

Comienza con mcp-core/src/main/java/io/modelcontextprotocol/util/Assert.java y busca en el repositorio reactor.util.annotation.Nullable y las anotaciones de nulabilidad relacionadas de Reactor. Comprueba la configuración de compilación para localizar la dependencia de JSpecify y, después, verifica que todas las anotaciones relevantes utilicen JSpecify y que el SDK se compile sin requerir las clases de anotaciones de javax.