Check for localhost in remote url

[Avish34]

Add check for localhost in remote url.
#274 (comment)

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.

Breaking Changes

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

[Avish34]

@tadasant / @domdomegg I see we have one testcase which says "allow localhost urls" in validation_test.go. Can you please help clarify what's the expectation here?

{ name: "localhost URLs allowed with any namespace", serverDetail: apiv0.ServerJSON{ Name: "com.example/test-server", Remotes: []model.Remote{ {URL: "http://localhost:3000/sse"}, }, }, expectError: false, },

[domdomegg]

happy to merge as-is and add the test as a follow-up, will hold open for a few hours so you can add the test to this one if you want :)

[domdomegg]

@Avish34 I think we probably don't want to enable localhost urls, that test is wrong and can be deleted. I think we'll figure out the latest with how we handle local servers in #345 (comment), for now we can disallow them here.

[domdomegg]

will fix the merge conflicts for this in a sec