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

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## 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/validators/validators.go

@@ -53,6 +53,16 @@ var (
 )
 
 func ValidateServerJSON(serverJSON *apiv0.ServerJSON) error {
+	// Validate schema version is provided and supported
+	// Note: Schema field is also marked as required in the ServerJSON struct definition
+	// for API-level validation and documentation
+	if serverJSON.Schema == "" {
+		return fmt.Errorf("$schema field is required")
+	}
+	if !strings.Contains(serverJSON.Schema, model.CurrentSchemaVersion) {
+		return fmt.Errorf("schema version %s is not supported. Please use schema version %s", serverJSON.Schema, model.CurrentSchemaVersion)
+	}
+
 	// Validate server name exists and format
 	if _, err := parseServerName(*serverJSON); err != nil {
 		return err