fix(api): preserve automatic networking on legacy instance updates

[chet]

Instance updates from clients that still send the deprecated InstanceNetworkConfig.auto field should keep working when Core already has the instance's auto_config. Today the allocation path translates that legacy field but the update path does not, so Core can mistake an omitted replacement field for a request to change immutable network configuration.

What this involves

• Teach update_instance_config to preserve the stored auto_config only for a legacy auto=true request that omits auto_config, before the RPC request is converted into InstanceConfig.
• Use the instance's existing configuration as the source of truth instead of looking the VPC back up through its HostInband network segment; Flat segments are allowed to be unbound.
• Keep explicit requests and the existing immutability checks authoritative -- don't overwrite a supplied auto_config, enable automatic networking on a non-auto instance, or accept explicit interfaces alongside legacy automatic networking.
• Add regression coverage for the compatible update and rejection cases, including an instance whose Flat segment has no VPC binding.