Ban 'latest' as a reserved version string (#413)

## Summary
- Adds validation to prevent servers from being published with version
string "latest"
- "latest" is now a reserved keyword that cannot be used as a version
string

## Implementation
- Added `ErrReservedVersionString` error constant
- Created `validateVersion` function to check for reserved version
strings
- Integrated validation into the package field validation flow
- Added comprehensive test coverage

---

This is so we can potentially support things like
https://github.com/modelcontextprotocol/registry/discussions/333 in
future more easily, as I imagine we might want to.

internal/validators/validators.go

@@ -72,6 +72,11 @@ func validatePackageField(obj *model.Package) error {
 		return ErrPackageNameHasSpaces
 	}
 
+	// Validate version string
+	if err := validateVersion(obj.Version); err != nil {
+		return err
+	}
+
 	// Validate runtime arguments
 	for _, arg := range obj.RuntimeArguments {
 		if err := validateArgument(&arg); err != nil {
@@ -95,6 +100,16 @@ func validatePackageField(obj *model.Package) error {
 	return nil
 }
 
+// validateVersion validates the version string
+// NB: we decided that we would not enforce strict semver for version strings
+func validateVersion(version string) error {
+	if version == "latest" {
+		return ErrReservedVersionString
+	}
+
+	return nil
+}
+
 // validateArgument validates argument details
 func validateArgument(obj *model.Argument) error {
 	if obj.Type == model.ArgumentTypeNamed {
@@ -144,12 +159,12 @@ func validateArgumentValueFields(name, value, defaultValue string) error {
 // collectAvailableVariables collects all available template variables from a package
 func collectAvailableVariables(pkg *model.Package) []string {
 	var variables []string
-	
+
 	// Add environment variable names
 	for _, env := range pkg.EnvironmentVariables {
 		variables = append(variables, env.Name)
 	}
-	
+
 	// Add runtime argument names and value hints
 	for _, arg := range pkg.RuntimeArguments {
 		if arg.Name != "" {
@@ -159,7 +174,7 @@ func collectAvailableVariables(pkg *model.Package) []string {
 			variables = append(variables, arg.ValueHint)
 		}
 	}
-	
+
 	// Add package argument names and value hints
 	for _, arg := range pkg.PackageArguments {
 		if arg.Name != "" {
@@ -169,7 +184,7 @@ func collectAvailableVariables(pkg *model.Package) []string {
 			variables = append(variables, arg.ValueHint)
 		}
 	}
-	
+
 	return variables
 }
 
@@ -193,7 +208,7 @@ func validatePackageTransport(transport *model.Transport, availableVariables []s
 			// Check if it's a template variable issue or basic URL issue
 			templateVars := extractTemplateVariables(transport.URL)
 			if len(templateVars) > 0 {
-				return fmt.Errorf("%w: template variables in URL %s reference undefined variables. Available variables: %v", 
+				return fmt.Errorf("%w: template variables in URL %s reference undefined variables. Available variables: %v",
 					ErrInvalidRemoteURL, transport.URL, availableVariables)
 			}
 			return fmt.Errorf("%w: %s", ErrInvalidRemoteURL, transport.URL)