Add method to get the MCP handler

[Randgalt]

Add method to get the MCP handler

Motivation and Context

This will allow setting custom MCP handlers that defer to the handler that is set by the framework

How Has This Been Tested?

Tests aren't really necessary as this is merely a simple getter

Breaking Changes

Adds a new method to McpStatelessServerTransport. As an alternative, we could add a default implementation that throws UnsupportedOperation

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

[Randgalt]

We think we can do what we want without this change. Closing for now.