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
<!-- Will users need to update their code or configurations? -->
**Note this can be considered breaking if clients have not included it
before.**

## 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>

cmd/publisher/commands/init.go

@@ -301,7 +301,7 @@ func createServerJSON(
 
 	// Create server structure
 	return apiv0.ServerJSON{
-		Schema:      "https://static.modelcontextprotocol.io/schemas/2025-09-29/server.schema.json",
+		Schema:      model.CurrentSchemaURL,
 		Name:        name,
 		Description: description,
 		Repository: model.Repository{

cmd/publisher/commands/publish.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 
 	apiv0 "github.com/modelcontextprotocol/registry/pkg/api/v0"
+	"github.com/modelcontextprotocol/registry/pkg/model"
 )
 
 func PublishCommand(args []string) error {
@@ -39,8 +40,8 @@ func PublishCommand(args []string) error {
 
 	// Check for deprecated schema and recommend migration
 	// Allow empty schema (will use default) but reject old schemas
-	if serverJSON.Schema != "" && !strings.Contains(serverJSON.Schema, "2025-09-29") {
-		return fmt.Errorf(`deprecated schema detected :%s.
+	if serverJSON.Schema != "" && !strings.Contains(serverJSON.Schema, model.CurrentSchemaVersion) {
+		return fmt.Errorf(`deprecated schema detected: %s.
 
 Migrate to the current schema format for new servers.