Completions capability flag

[PederHP]

Motivation and Context

The completions capability is the only one without a ServerCapabilities flag, which doesn't make sense as not all servers will support it, and clients should be able to query that up front to avoid calling the method. See #157.

How Has This Been Tested?

Not yet no. It's relatively trivial and aligns with the capability flags. I'll implement this in mcpdotnet if/when the PR is merged.

Breaking Changes

In principle yes, as servers should now expose the completions capability explicitly. After this change, clients that query servers may assume that a server not exposing the capability does not support it. In practice, this capability is so tightly coupled to server documentation (as there is no mechanism for servers to communicate what completion parameters they support) that clients will likely be hardcoded to use completions with specific servers, making this a breaking change in theory only.

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]

Thank you! Sorry for the delayed review.