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?
<!-- Have you tested this in a real application? Which scenarios were
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
<!-- Add any other context, implementation notes, or design decisions
-->

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

internal/importer/importer_test.go

@@ -26,7 +26,7 @@ func TestImportService_LocalFile(t *testing.T) {
 			Schema:      model.CurrentSchemaURL,
 			Name:        "io.github.test/test-server-1",
 			Description: "Test server 1",
-			Repository: model.Repository{
+			Repository: &model.Repository{
 				URL:    "https://github.com/test/repo1",
 				Source: "github",
 				ID:     "123",
@@ -69,7 +69,7 @@ func TestImportService_HTTPFile(t *testing.T) {
 			Schema:      model.CurrentSchemaURL,
 			Name:        "io.github.test/http-test-server",
 			Description: "HTTP test server",
-			Repository: model.Repository{
+			Repository: &model.Repository{
 				URL:    "https://github.com/test/http-repo",
 				Source: "github",
 				ID:     "456",