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/nuget.go

@@ -2,6 +2,7 @@ package registries
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -11,6 +12,11 @@ import (
 	"github.com/modelcontextprotocol/registry/pkg/model"
 )
 
+var (
+	ErrMissingIdentifierForNuget = errors.New("package identifier is required for NuGet packages")
+	ErrMissingVersionForNuget    = errors.New("package version is required for NuGet packages")
+)
+
 // ValidateNuGet validates that a NuGet package contains the correct MCP server name
 func ValidateNuGet(ctx context.Context, pkg model.Package, serverName string) error {
 	// Set default registry base URL if empty
@@ -18,6 +24,10 @@ func ValidateNuGet(ctx context.Context, pkg model.Package, serverName string) er
 		pkg.RegistryBaseURL = model.RegistryURLNuGet
 	}
 
+	if pkg.Identifier == "" {
+		return ErrMissingIdentifierForNuget
+	}
+
 	// Validate that the registry base URL matches NuGet exactly
 	if pkg.RegistryBaseURL != model.RegistryURLNuGet {
 		return fmt.Errorf("registry type and base URL do not match: '%s' is not valid for registry type '%s'. Expected: %s",
@@ -29,7 +39,7 @@ func ValidateNuGet(ctx context.Context, pkg model.Package, serverName string) er
 	lowerID := strings.ToLower(pkg.Identifier)
 	lowerVersion := strings.ToLower(pkg.Version)
 	if lowerVersion == "" {
-		return fmt.Errorf("NuGet package validation requires a specific version, but none was provided")
+		return ErrMissingVersionForNuget
 	}
 
 	// Try to get README from the package