fix(spec): registryType, identifier, version should be required fields (#548)

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## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->

This seems to be a regresion after rename, those fields were originally
required:


8d7e471fd3/docs/server-registry-api/openapi.yaml (L247-L252)

In
https://static.modelcontextprotocol.io/schemas/2025-09-16/server.schema.json,
those fields are also required.

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

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
No

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] 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)
- [x] 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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-->

docs/reference/api/openapi.yaml

@@ -319,6 +319,10 @@ components:
 
     Package:
       type: object
+      required:
+        - registryType
+        - identifier
+        - version
       properties:
         registryType:
           type: string