Refactor: Extract Capabilities object

[kfischer-okarin]

I extracted a Capabilities object, which allows setting the capabilities from the server incrementally and encapsulates the actual calculation of the final capabilities hash.

Motivation and Context

The current determine_capabilities approach would need to become a bit more complex to support all the sub-capabilities without interfering with the already supported auto-determination. This would need a bit of custom bookkeeping which if kept in the server would cause quite a bit of cognitive noise. So I extracted it to its own object with an idempotent "support_..." API.

This is originally thought as a refactor to prepare for easy implementation of the Server#support_notifications method I proposed in #33 - but even if that API should not be adopted after all, I still think it's nice to hide all that capability determination logic in its own object.

How Has This Been Tested?

I did not test this manually, but the existing unit tests still pass.

Breaking Changes

No

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

None

[kfischer-okarin]

BTW I had some help from Claude Code (as pointed out in the commit, too), especially with writing out all the Unit Tests - feel free to delete some of them if you find them too extensive