Use constants instead of hardcoded strings (#330)

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## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR updates some of the hardcoded references to public
constants.


## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
Locally
## Breaking Changes
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No

## Types of changes
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the boxes that apply: -->
- [x] 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
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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
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Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

internal/validators/validators.go

@@ -168,7 +168,7 @@ func validatePackage(pkg *model.Package) error {
 	registryType := strings.ToLower(pkg.RegistryType)
 
 	// For direct download packages (mcpb or direct URLs)
-	if registryType == "mcpb" ||
+	if registryType == model.RegistryTypeMCPB ||
 		strings.HasPrefix(pkg.Identifier, "http://") || strings.HasPrefix(pkg.Identifier, "https://") {
 		parsedURL, err := url.Parse(pkg.Identifier)
 		if err != nil {
@@ -178,7 +178,7 @@ func validatePackage(pkg *model.Package) error {
 		host := strings.ToLower(parsedURL.Host)
 
 		// For MCPB packages, validate they're from allowed hosts
-		if registryType == "mcpb" {
+		if registryType == model.RegistryTypeMCPB {
 			return validateMCPBPackage(host)
 		}