`modelctl debug` exits 0 on non-existent subcommand

### Bug description

Found by @mrmara in https://github.com/canonical/nomic-embed-text-v1.5-snap/pull/1#discussion_r3658061623

### To reproduce

```
~/inference-snaps-workspace/inference-snaps-cli/cmd/modelctl$ go run . debug validate-engines ../../..
Developer/debugging commands

Usage:
cli debug [command]

Available Commands:
lint-package Validate engines, models, and runtimes manifest files
select-engine Test which engine will be chosen
chat Start the chat CLI providing connection parameters

Flags:
-h, --help help for debug

Global Flags:
-v, --verbose Enable verbose logging

Use "cli debug [command] --help" for more information about a command.
~/inference-snaps-workspace/inference-snaps-cli/cmd/modelctl$ echo $?
0
```

### Snap version

Tested with:
* v2.0.0-beta.8
* v2.0.0-beta.11
* commit 6a23692cd31245ed47dfb793bac158900bfe50b2

### System information

_No response_

Start in cmd/modelctl and run the reported `go run . debug validate-engines ../../..` reproduction. Trace how the unknown `debug` subcommand is handled and verify the command exits nonzero while retaining the existing usage output. Add or update the relevant test if one is found nearby, then rerun the reproduction to confirm the exit status.

Written by the indexing model from the issue text.