Allow stable protocol specification version "2025-11-25"

[koic]

Motivation and Context

The MCP specification dated 2025-11-25 has been released:

• https://modelcontextprotocol.io/specification/2025-11-25
• https://blog.modelcontextprotocol.io/posts/2025-11-25-first-mcp-anniversary

This PR allows '2025-11-25' to be specified for the :protocol_version keyword argument in MCP::Configuration.new.

How Has This Been Tested?

The tests have been updated and are now passing.

Breaking Changes

None.

Types of changes

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

Checklist

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

[koic]

@modelcontextprotocol/ruby-sdk Can you review this PR? So I will proceed with the follow-up 2025-11-25 specification feature support work.

[koic]

Please review and merge #190 first, since it is a bug fix. This PR needs to be updated based on #190.