fix: Ensure the correct capability is checked for completion/complete

[morinokami]

Motivation and Context

Fixed an issue where prompts capability was being checked instead of completions in assertCapabilityForMethod. Now it properly checks the completions capability.

How Has This Been Tested?

Updated the test to ensure that any request for a capability not listed in the server's capabilities results in an error.

Breaking Changes

N/A

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]

Oops, thanks!