fly config validate reports an invalid [[vm]] memory value as valid #5262

Describe the bug

fly config validate accepts a config whose machine conversion fails, such as an unparsable memory value in a [[vm]] section. The command prints "Configuration is valid", and the error appears only later, when fly deploy converts the config to a machine config.

Steps to reproduce

1. Create a fly.toml with:

   app = "foo"
   
   [[vm]]
     memory = "not-a-size"

2. Run fly config validate.

Expected

Validation fails and reports that converting to a machine in process group app will fail, with the parse error.

Actual

✓ Configuration is valid

The cause is in validateMachineConversion in internal/appconfig/validation.go. It checks the named return value err, which is always nil at that point, instead of the error returned by ToMachineConfig.