Fix method not found error msg for server

[CrazyHZM]

Motivation and Context

The server originally did not support the root/list method, and the "method not found" error message is invalid.

How Has This Been Tested?

Breaking Changes

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

[tzolov]

Thanks for cleaning this @CrazyHZM