allow `nomad config validate` to include command line flags in validation

### Nomad version
Output from `nomad version`
Nomad v1.8.4
BuildDate 2024-09-17T20:18:34Z
Revision 22ab32e6cff66cf52f5e3f115b93de776bc09583
### Operating system and Environment details
macOS 15.3.1 (though issue is not OS-specific)
### Issue
Running `nomad config validate` doesn't work on a `config.hcl` file that contains both `client {}` and `server {}` blocks.
My understanding is that this is a valid configuration (we use it in production), and just launch the agent with the `-client` or `-server` flag as appropriate for the machine.

### Reproduction steps

#### Expected Result
Config is validated
#### Actual Result
```
Must specify either server, client or dev mode for the agent.
Configuration is invalid
```
### Job file (if appropriate)

### Nomad Server logs (if appropriate)

### Nomad Client logs (if appropriate)

Start with the implementation of the `nomad config validate` command and reproduce the failure using a configuration containing both `client {}` and `server {}` blocks. Trace how agent mode is selected and determine how command-line flags should participate in validation; done means the configuration validates successfully when the appropriate mode flag is supplied.

Written by the indexing model from the issue text.