fix: Handle port-position template variables in URL validation

When validating URLs with template variables in the port position
(e.g., http://localhost:{--port}/mcp), the validator was incorrectly
replacing the template variable with the string "placeholder", which
caused URL parsing to fail since ports must be numeric.

This commit fixes the issue by:
- Detecting when a template variable appears in a port position
  (after a colon and before a slash or end of string)
- Replacing port-position template variables with a numeric placeholder
  "8080" instead of "placeholder"
- Adding a new error constant ErrInvalidPackageTransportURL to
  distinguish package transport errors from remote transport errors
- Updating validatePackageTransport to use the correct error constant

The fix allows named arguments like "--port" to be properly referenced
in package transport URLs, enabling examples like:
```json
{
  "transport": {
    "type": "streamable-http",
    "url": "http://localhost:{--port}/mcp"
  },
  "packageArguments": [
    {
      "type": "named",
      "name": "--port",
      "default": "3000"
    }
  ]
}
```

Fixes validation of Example 15 in generic-server-json.md (line 732)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: tadasant

internal/validators/constants.go

@@ -13,8 +13,9 @@ var (
 	ErrReservedVersionString = errors.New("version string 'latest' is reserved and cannot be used")
 	ErrVersionLooksLikeRange = errors.New("version must be a specific version, not a range")
 
-	// Remote validation errors
-	ErrInvalidRemoteURL = errors.New("invalid remote URL")
+	// Transport validation errors
+	ErrInvalidPackageTransportURL = errors.New("invalid package transport URL")
+	ErrInvalidRemoteURL           = errors.New("invalid remote URL")
 
 	// Registry validation errors
 	ErrUnsupportedRegistryBaseURL   = errors.New("unsupported registry base URL")

internal/validators/utils.go

@@ -61,7 +61,13 @@ func replaceTemplateVariables(rawURL string) string {
 		result = strings.ReplaceAll(result, placeholder, replacement)
 	}
 
-	// Handle any remaining {variable} patterns with generic placeholder
+	// Handle any remaining {variable} patterns with context-appropriate placeholders
+	// If the variable is in a port position (after a colon in the host), use a numeric placeholder
+	// Pattern: :/{variable} or :{variable}/ or :{variable} at end
+	portRe := regexp.MustCompile(`:(\{[^}]+\})(/|$)`)
+	result = portRe.ReplaceAllString(result, ":8080$2")
+
+	// Replace any other remaining {variable} patterns with generic placeholder
 	re := regexp.MustCompile(`\{[^}]+\}`)
 	result = re.ReplaceAllString(result, "placeholder")
 

internal/validators/validators.go

@@ -409,9 +409,9 @@ func validatePackageTransport(transport *model.Transport, availableVariables []s
 			templateVars := extractTemplateVariables(transport.URL)
 			if len(templateVars) > 0 {
 				return fmt.Errorf("%w: template variables in URL %s reference undefined variables. Available variables: %v",
-					ErrInvalidRemoteURL, transport.URL, availableVariables)
+					ErrInvalidPackageTransportURL, transport.URL, availableVariables)
 			}
-			return fmt.Errorf("%w: %s", ErrInvalidRemoteURL, transport.URL)
+			return fmt.Errorf("%w: %s", ErrInvalidPackageTransportURL, transport.URL)
 		}
 		return nil
 	default: