Fix method name in client cancellation send

[efritz]

The server registers a notification handler on CancelledNotificationSchema, but is never called on client cancellation because of a mismatch in the method name. This fixes that mismatch.

Motivation and Context

Cancellation between server <-> client implementations does not seem to function correctly. There's no propagation of an abort signal from the client to the server.

How Has This Been Tested?

I made this change in the dependency tree of a local application and saw that the server signal is properly canceled after the change.

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

[jspahrsummers]

Very good catch, thank you!