Servers: Variables do not support non-enums

[atombender]

Given a server list like this:

servers:
- url: https://{hostPort}/v{version}
  x-ogen-server-name: development
  variables:
    version:
      description: API version
      default: UNKNOWN  # Ogen requires a dummy default here
    hostPort:
      description: Host and port
      default: UNKNOWN  # Ogen requires a dummy default here

This will generate the following validation code:

func (s DevelopmentServer) Build() (string, error) {
	zeroOr := func(s string, def string) string {
		if s == "" {
			return def
		}
		return s
	}
	s.HostPort = zeroOr(s.HostPort, "UNKNOWN")
	// Validate "hostPort"
	switch s.HostPort {
	default:
		return "", errors.Errorf("param %q: unexpected value %q", "hostPort", s.HostPort)
	}
	...

This means it's not possible to create structs with arbitrary data:

srv := DevelopmentServer{
	HostPort: "localhost:8080",
	Version: "2024-06-29",
}

as this will fail with:

param "hostPort": unexpected value "localhsot:8080":

This is in violation of the OpenAPI spec, which says the enum is optional (emphasis mine):

An enumeration of string values to be used if the substitution options are from a limited set

The validation template should just skip non-set values, e.g.:

	{{- if $p.Spec.Enum }}
	// Validate {{ quote $p.Spec.Name }}
	switch s.{{ $p.Name }} {
	{{- range $e := $p.Spec.Enum }}
	case {{ quote $e }}:
	{{- end }}
	default:
		return "", errors.Errorf("param %q: unexpected value %q", {{ quote $p.Spec.Name }}, s.{{ $p.Name }})
	}
	{{- end }}