gsutil mb fails with incorrect error

When I run a valid `gsutil mb` command, I get an invalid error:

```
CommandException: "mb" command does not support "file://" URLs. Did you mean to use a gs:// URL?
```

I _did_ use a `gs://` URL.

Start with the gsutil mb command entry point and reproduce the reported URL classification error using a valid gs:// URL. The issue does not include the exact command, environment, or a named test, so first determine the missing reproduction details; done means valid gs:// input no longer produces the file:// error and coverage verifies the behavior.

Written by the indexing model from the issue text.