fix: Ensure completions capability is registered on setCompletionRequestHandler call

[fredericbarthelet]

Advertising support for completions in servers where a completionRequestHandler has been defined - meaning, when at least one resource or one prompt has been registered.

Just like:

• registerCapabilities is called to add tools support on setToolRequestHandlers call
• registerCapabilities is called to add resources support on setResourceRequestHandlers call
• registerCapabilities is called to add prompts support on setPromptRequestHandlers call

this PR calls registerCapabilities to turn completions support on whenever setCompletionRequestHandler is called

Motivation and Context

While implementing modelcontextprotocol/inspector#441, I found out that the only way to advertise completions support was defining it manually in Server constructor argument.

How Has This Been Tested?

I've tested this both with an application with a ResourceTemplate and a prompt with completable argument.

Breaking Changes

None

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