No automatic completion support unless needed

[cliffhall]

Description

• In completable.ts
  • add isCompletable function, a runtime type guard to detect Completable-wrapped Zod types
• In mcp.test.ts
  • Added tests
    • "should not advertise support for completion when a resource template without a complete callback is defined"
    • "should not advertise support for completion when a prompt without a completable argument is defined"
• In mcp.ts
  • in handlePromptCompletion method,
    • fix warning "Invalid 'instanceof' check: 'field' has type that is not related to 'Completable'" by using the new isCompletable function from completable.ts
  • in setResourceRequestHandlers method
    • remove unconditional call to setCompletionRequestHandler - supporting resources does not automatically mean that resource template completion is supported
  • in setPromptRequestHandlers method
    • remove call to setCompletionRequestHandler - supporting prompts does not automatically mean that prompt argument completion is supported
  • in _createRegisteredResourceTemplate method
    • check if the resource template being registered has a complete callback,
    • if so, call setCompletionRequestHandler
  • in _createRegisteredPrompt method
    • check if any argument of the prompt has a Completable schema
    • if so, call setCompletionRequestHandler

Motivation and Context

@evalstate discovered that when registering a prompt or resource template, it was automatically setting a completion request handler and advertising the completions capability. However in order to support completions, at least one argument of a prompt must have a Completable schema or one argument of a resource template must have a completeCallback.

In his case, he was not supporting completions on his prompts and resources, and was concerned that clients would send unnecessary completion requests because of this automatic advertisement of completions.

This PR checks each registered prompt and resource template, and only does automatic advertisement and listening for completion requests if completable argument is found.

How Has This Been Tested?

• Existing tests that auto advertisement is triggered by prompts and resource templates with completable arguments still pass:
  • "should advertise support for completion when a resource template with a complete callback is defined"
  • "should advertise support for completion when a prompt with a completable argument is defined"
• New tests that auto advertisement is not triggered by prompts and resource templates with no completable arguments also pass:
  • "should not advertise support for completion when a resource template without a complete callback is defined"
  • "should not advertise support for completion when a prompt without a completable argument is defined"

Breaking Changes

Nope. Existing behavior for auto-advertisement of completions will still allow completable arguments to be completed. If no completable prompt or resource template arguments exist, then the previous behavior of allowing completion requests was a bug and will now be defeated.

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