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>

pkg/model/constants.go

@@ -34,3 +34,11 @@ const (
 	RuntimeHintDocker = "docker"
 	RuntimeHintDNX    = "dnx"
 )
+
+// Schema versions
+const (
+	// CurrentSchemaVersion is the current supported schema version date
+	CurrentSchemaVersion = "2025-09-29"
+	// CurrentSchemaURL is the full URL to the current schema
+	CurrentSchemaURL = "https://static.modelcontextprotocol.io/schemas/" + CurrentSchemaVersion + "/server.schema.json"
+)