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_test.go

@@ -21,6 +21,30 @@ func TestValidateOCI_RealPackages(t *testing.T) {
 		errorMessage string
 		registryURL  string
 	}{
+		{
+			name:         "empty package identifier should fail",
+			packageName:  "",
+			version:      "latest",
+			serverName:   "com.example/test",
+			expectError:  true,
+			errorMessage: "package identifier is required for OCI packages",
+		},
+		{
+			name:         "empty package version should fail",
+			packageName:  "test-image",
+			version:      "",
+			serverName:   "com.example/test",
+			expectError:  true,
+			errorMessage: "package version is required for OCI packages",
+		},
+		{
+			name:         "both empty identifier and version should fail with identifier error first",
+			packageName:  "",
+			version:      "",
+			serverName:   "com.example/test",
+			expectError:  true,
+			errorMessage: "package identifier is required for OCI packages",
+		},
 		{
 			name:         "non-existent image should fail",
 			packageName:  generateRandomImageName(),