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.

[steven-ji]

I would like to work on this issue.

I verified that the current main branch still has six mcp-core imports of Reactor's deprecated Nullable annotation and that there is no active implementation PR.

I will keep the change focused on replacing those existing imports with JSpecify and adding only the direct dependency needed by mcp-core. I will not introduce package-wide @NullMarked semantics or unrelated nullness refactoring. I will validate the dependency graph, compilation, formatting, and the full mcp-core test suite.

If you have already started an implementation, please let me know so I can avoid duplicate work.