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/npm.go

@@ -3,6 +3,7 @@ package registries
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"net/url"
@@ -11,6 +12,11 @@ import (
 	"github.com/modelcontextprotocol/registry/pkg/model"
 )
 
+var (
+	ErrMissingIdentifierForNPM = errors.New("package identifier is required for NPM packages")
+	ErrMissingVersionForNPM    = errors.New("package version is required for NPM packages")
+)
+
 // NPMPackageResponse represents the structure returned by the NPM registry API
 type NPMPackageResponse struct {
 	MCPName string `json:"mcpName"`
@@ -24,7 +30,7 @@ func ValidateNPM(ctx context.Context, pkg model.Package, serverName string) erro
 	}
 
 	if pkg.Identifier == "" {
-		return fmt.Errorf("package identifier is required for NPM packages")
+		return ErrMissingIdentifierForNPM
 	}
 
 	// we need version to look up the package metadata
@@ -32,7 +38,7 @@ func ValidateNPM(ctx context.Context, pkg model.Package, serverName string) erro
 	// and we won't be able to validate the mcpName field
 	// against the server name
 	if pkg.Version == "" {
-		return fmt.Errorf("package version is required for NPM packages")
+		return ErrMissingVersionForNPM
 	}
 
 	// Validate that the registry base URL matches NPM exactly