Enforce schema validation on the publish and edit API level (#598)

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

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR:
* Enforces schema validation on the API level (requests without schema
or older schema will fail)
* Updates the unit tests and examples to match that

## How Has This Been Tested?
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tested? -->

## Breaking Changes
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**Note this can be considered breaking if clients have not included it
before.**

## Types of changes
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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
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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>

internal/importer/importer_test.go

@@ -23,6 +23,7 @@ func TestImportService_LocalFile(t *testing.T) {
 	tempFile := "/tmp/test_import_seed.json"
 	seedData := []*apiv0.ServerJSON{
 		{
+			Schema:      model.CurrentSchemaURL,
 			Name:        "io.github.test/test-server-1",
 			Description: "Test server 1",
 			Repository: model.Repository{
@@ -65,6 +66,7 @@ func TestImportService_HTTPFile(t *testing.T) {
 	// Create a test HTTP server
 	seedData := []*apiv0.ServerJSON{
 		{
+			Schema:      model.CurrentSchemaURL,
 			Name:        "io.github.test/http-test-server",
 			Description: "HTTP test server",
 			Repository: model.Repository{
@@ -115,11 +117,13 @@ func TestImportService_RegistryPagination(t *testing.T) {
 	// Setup source registry with test data
 	sourceServers := []*apiv0.ServerJSON{
 		{
+			Schema:      model.CurrentSchemaURL,
 			Name:        "com.source/server-1",
 			Description: "Source server 1",
 			Version:     "1.0.0",
 		},
 		{
+			Schema:      model.CurrentSchemaURL,
 			Name:        "com.source/server-2",
 			Description: "Source server 2",
 			Version:     "1.0.0",