[Server] Add completion/complete handler, centralize provider logic, and move completion capability

[CodeWithKyrian]

This PR introduces handler for completion/complete method through a new Completer and CompletionCompleteHandler. Completion logic has also been moved reflecting its broader scope across both prompts and resource templates.

What's Changed

• Added CompletionCompleteHandler and wired it into JsonRpcHandler::make.
• Moved Capability/Prompt/Completion to Capability/Completion.
• Added Completer: resolves reference and provider, returns paged CompletionCompleteResult.
• Updated Builder/Discoverer/examples/tests to new namespaces.
• Removed complete() from PromptReference and ResourceTemplateReference.

Breaking Changes

No direct impact on the public API and usage.

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

[chr-hertel]

Nice one, thanks! 👍 Needs a rebase after #98 tho