Migrate null annotations to jspecify

[michaelboyles]

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

java-sdk/mcp-core/src/main/java/io/modelcontextprotocol/util/Assert.java

Line 28 in fd00498

public static void notEmpty(@Nullable Collection<?> collection, String message) {

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.