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/oci.go

@@ -13,6 +13,11 @@ import (
 	"github.com/modelcontextprotocol/registry/pkg/model"
 )
 
+var (
+	ErrMissingIdentifierForOCI = errors.New("package identifier is required for OCI packages")
+	ErrMissingVersionForOCI    = errors.New("package version is required for OCI packages")
+)
+
 const (
 	dockerIoAPIBaseURL = "https://registry-1.docker.io"
 	ghcrAPIBaseURL     = "https://ghcr.io"
@@ -80,6 +85,15 @@ func ValidateOCI(ctx context.Context, pkg model.Package, serverName string) erro
 		pkg.RegistryBaseURL = model.RegistryURLDocker
 	}
 
+	if pkg.Identifier == "" {
+		return ErrMissingIdentifierForOCI
+	}
+
+	// we need version (tag) to look up the image manifest
+	if pkg.Version == "" {
+		return ErrMissingVersionForOCI
+	}
+
 	// Validate that the registry base URL is supported
 	if err := validateRegistryURL(pkg.RegistryBaseURL); err != nil {
 		return err
@@ -258,7 +272,6 @@ func getRegistryAuthToken(ctx context.Context, client *http.Client, config *Regi
 	return authResp.Token, nil
 }
 
-
 // getSpecificManifest retrieves a specific manifest for multi-arch images
 func getSpecificManifest(ctx context.Context, client *http.Client, registryConfig *RegistryConfig, namespace, repo, digest string) (*OCIManifest, error) {
 	manifestURL := fmt.Sprintf("%s/v2/%s/%s/manifests/%s", registryConfig.APIBaseURL, namespace, repo, digest)