Make mcpb package version optional and fix schema validation (#686)

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## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR:
* Makes mcpb package version property optional
* Bumps the schema and reflects that in the code
* Makes sure the publisher tests are being covered by doing `make check`

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

## Breaking Changes
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## 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>

tests/integration/run.sh

@@ -24,7 +24,8 @@ cleanup() {
 
 go build -o ./bin/publisher ./cmd/publisher
 
-docker build -t registry .
+# Build the registry image with both tags, forcing a rebuild to ensure latest code
+docker build --no-cache -t registry -t modelcontextprotocol/registry:dev .
 
 trap cleanup EXIT