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_test.go

@@ -20,6 +20,30 @@ func TestValidateMCPB(t *testing.T) {
 		expectError  bool
 		errorMessage string
 	}{
+		{
+			name:         "empty package identifier should fail",
+			packageName:  "",
+			serverName:   "com.example/test",
+			fileSHA256:   "abc123ef4567890abcdef1234567890abcdef1234567890abcdef1234567890",
+			expectError:  true,
+			errorMessage: "package identifier is required for MCPB packages",
+		},
+		{
+			name:         "empty file SHA256 should fail",
+			packageName:  "https://github.com/example/server/releases/download/v1.0.0/server.mcpb",
+			serverName:   "com.example/test",
+			fileSHA256:   "",
+			expectError:  true,
+			errorMessage: "must include a fileSha256 hash for integrity verification",
+		},
+		{
+			name:         "both empty identifier and file SHA256 should fail with file SHA256 error first",
+			packageName:  "",
+			serverName:   "com.example/test",
+			fileSHA256:   "",
+			expectError:  true,
+			errorMessage: "must include a fileSha256 hash for integrity verification",
+		},
 		{
 			name:        "valid MCPB package should pass",
 			packageName: "https://github.com/domdomegg/airtable-mcp-server/releases/download/v1.7.2/airtable-mcp-server.mcpb",