Extra validation for registries (#484)

<!-- Provide a brief summary of your changes -->
Package identifier and version are required for some of the registries
else we won't be able to pull the meta data to validate the packages.

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
This is in continuation to
https://github.com/modelcontextprotocol/registry/pull/424

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
Unit test.

## 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: -->
- [ ] 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
<!-- Add any other context, implementation notes, or design decisions
-->

Co-authored-by: Avish Porwal <avishporwal@microsoft.com>

internal/validators/registries/mcpb.go

@@ -12,10 +12,19 @@ import (
 	"github.com/modelcontextprotocol/registry/pkg/model"
 )
 
+var (
+	ErrMissingIdentifierForMCPB = fmt.Errorf("package identifier is required for MCPB packages")
+	ErrMissingFileSHA256ForMCPB = fmt.Errorf("must include a fileSha256 hash for integrity verification")
+)
+
 func ValidateMCPB(ctx context.Context, pkg model.Package, _ string) error {
 	// MCPB packages must include a file hash for integrity verification
 	if pkg.FileSHA256 == "" {
-		return fmt.Errorf("MCPB package must include a fileSha256 hash for integrity verification")
+		return ErrMissingFileSHA256ForMCPB
+	}
+
+	if pkg.Identifier == "" {
+		return ErrMissingIdentifierForMCPB
 	}
 
 	err := validateMCPBUrl(pkg.Identifier)