Make Repository field properly optional in server.json (#717)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The Repository field in ServerJSON was not properly omitting empty
values
when marshaling to JSON. Even when a server had no repository
configured,
the JSON would include `"repository": {"url": "", "source": ""}` due to:

1. Repository struct fields (URL, Source) lacking omitempty tags
2. Repository being a value type instead of pointer in ServerJSON

This caused validation warnings for servers without repository URLs.

Changes:
- Add omitempty tags to Repository.URL and Repository.Source fields
- Change ServerJSON.Repository from model.Repository to
*model.Repository
- Update validateRepository to handle nil pointers
- Update all code creating Repository instances to use pointers

Now, servers without repository information correctly omit the field
entirely from JSON output, eliminating spurious validation warnings.
## How Has This Been Tested?
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tested? -->

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] 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
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] 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
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-->

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

pkg/api/v0/types.go

@@ -36,7 +36,7 @@ type ServerJSON struct {
 	Name        string            `json:"name" minLength:"3" maxLength:"200" pattern:"^[a-zA-Z0-9.-]+/[a-zA-Z0-9._-]+$" doc:"Server name in reverse-DNS format. Must contain exactly one forward slash separating namespace from server name." example:"io.github.user/weather"`
 	Description string            `json:"description" minLength:"1" maxLength:"100" doc:"Clear human-readable explanation of server functionality." example:"MCP server providing weather data and forecasts via OpenWeatherMap API"`
 	Title       string            `json:"title,omitempty" minLength:"1" maxLength:"100" doc:"Optional human-readable title or display name for the MCP server." example:"Weather API"`
-	Repository  model.Repository  `json:"repository,omitempty" doc:"Optional repository metadata for the MCP server source code."`
+	Repository  *model.Repository `json:"repository,omitempty" doc:"Optional repository metadata for the MCP server source code."`
 	Version     string            `json:"version" doc:"Version string for this server. SHOULD follow semantic versioning." example:"1.0.2"`
 	WebsiteURL  string            `json:"websiteUrl,omitempty" format:"uri" doc:"Optional URL to the server's homepage, documentation, or project website." example:"https://modelcontextprotocol.io/examples"`
 	Icons       []model.Icon      `json:"icons,omitempty" doc:"Optional set of sized icons that the client can display in a user interface."`