fix: pass fetchfn parameter to registerClient and refreshAuthorizatio…

[arjunkmrm]

Fixed missing fetchFn parameter in registerClient and refreshAuthorization function calls in the auth flow

Motivation and Context

The authInternal function accepts a fetchFn parameter and correctly passes it to most HTTP calls (discoverOAuthProtectedResourceMetadata, discoverAuthorizationServerMetadata, exchangeAuthorization), but was missing it in calls to registerClient and refreshAuthorization. This caused these functions to fall back to the global fetch instead of using any custom fetchFn provided, creating inconsistent behavior in scenarios requiring custom network configurations, testing with mock fetch functions, or environments where global fetch might not be available.

How Has This Been Tested?

Tested in our application at Smithery (https://smithery.ai)

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

[pcarleton]

LGTM, thanks!