Check for localhost in remote url (#355)

Add check for localhost in remote url.

https://github.com/modelcontextprotocol/registry/discussions/274#discussioncomment-14124511

## Motivation and Context
We don't want authors to publish server with remote url set as
localhost.

## How Has This Been Tested?
It has been tested using UTs.

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] 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

---------

Co-authored-by: Avish Porwal <avishporwal@microsoft.com>
Co-authored-by: Adam Jones <domdomegg+git@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>

internal/validators/utils.go

@@ -43,7 +43,7 @@ func IsValidURL(rawURL string) bool {
 		return false
 	}
 
-	if u.Host == "" {
+	if u.Host == "" || u.Hostname() == "localhost" {
 		return false
 	}
 	return true