feat: Add completions capability in ServerCapabilitiesSchema

[morinokami]

Motivation and Context

Added completions to ServerCapabilitiesSchema in accordance with spec update #173, which introduced the completions capability.

Related: #268

How Has This Been Tested?

Confirmed visually that completions is present in the ServerCapabilities type. As this is an optional property, it is not expected to introduce a breaking change.

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]

Thanks!