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.

Start with mcp-core/src/main/java/io/modelcontextprotocol/util/Assert.java and search the repository for reactor.util.annotation.Nullable and related Reactor null annotations. Check the build configuration for the JSpecify dependency, then verify that all relevant annotations use JSpecify and that the SDK builds without requiring the javax annotation classes.