Migrate the existing mcp-core nullable annotations from Reactor's deprecated
reactor.util.annotation.Nullable to org.jspecify.annotations.Nullable.

The change adds JSpecify as a direct compile dependency of mcp-core, replaces the
five annotations that are actively used, and removes one unused Reactor Nullable
import. It does not introduce package-wide @NullMarked semantics or unrelated
nullness changes.

Motivation and Context

Reactor's Nullable annotation is deprecated and references legacy
javax.annotation.meta types. Downstream compilation can therefore report missing
annotation classes even though the SDK itself does not otherwise require them.

JSpecify provides the intended type-use nullness metadata without the legacy
javax.annotation dependency. Version 1.0.0 is also the version already present in
the repository's resolved test dependency graph.

Fixes #1066

How Has This Been Tested?

Before the change, compiling mcp-core reproduced the warning:

unknown enum constant javax.annotation.meta.When.MAYBE
class file for javax.annotation.meta.When not found

After the migration:

• ./mvnw -pl mcp-core clean test
  • 385 tests passed
  • 0 failures
  • 0 errors
  • the legacy javax.annotation.meta.When.MAYBE warning is absent
• ./mvnw -pl mcp-core -DskipTests package
  • package and source JAR generation passed
• ./mvnw -pl mcp-core dependency:tree -Dincludes=org.jspecify:jspecify
  • resolves org.jspecify:jspecify:1.0.0 as a direct compile dependency
• ./mvnw -pl mcp-core dependency:analyze-only
  • JSpecify is not reported as unused
  • unrelated pre-existing dependency-analysis warnings remain
• Inspected the flattened POM and OSGi manifest:
  • flattened POM contains JSpecify 1.0.0 with compile scope
  • OSGi imports org.jspecify.annotations with range [1.0,2)
  • OSGi no longer imports reactor.util.annotation
• Inspected the packaged bytecode for all five annotated public types; each references
  org/jspecify/annotations/Nullable
• git diff --check origin/main...HEAD

Full reactor validation also passed locally:

• env npm_config_cache=/private/tmp/java-sdk-1066-npm-cache ./mvnw clean test "-DsurefireArgLine=-Duser.language=en -Duser.country=US"
  • all 11 reactor modules succeeded
  • 958 tests passed
  • 0 failures, 0 errors, 0 skipped

Breaking Changes

No Java method signature or binary behavior changes. The public nullness annotation
metadata intentionally changes from Reactor Nullable to JSpecify Nullable.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

• The dependency is direct rather than provided so downstream consumers receive the
  annotation type referenced by the SDK's public bytecode.
• disclosure.txt is included as required by the repository contribution policy.