Handle SSH access failure (exit 255) in is_dispatch_aware with actionable error

[Copilot]

In Juju 4+, SSH access is not configured by default. is_dispatch_aware calls juju ssh and only handled exit code 1 (file not found); exit code 255 (SSH connection failure) was re-raised as a raw CalledProcessError, producing a noisy, unhelpful traceback.

Changes

• jhack/helpers.py — is_dispatch_aware: Catch CalledProcessError with returncode == 255 and raise a RuntimeError with a clear message and step-by-step instructions to add SSH keys to Juju.

RuntimeError: SSH access to 'hello-juju/0' failed (exit code 255).
SSH is not configured by default in Juju 4+.
To fix this, add your SSH key to Juju:
  1. If you don't have an SSH key yet: ssh-keygen -b 4096
  2. juju add-ssh-key "$(cat ~/.ssh/id_rsa.pub)"

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Jhack exception if no SSH access to Juju unit (default in Juju 4+)</issue_title>
<issue_description>### Bug Description

Hi,

usage jhack on default Juju 4 deployment generates a complex trace due to lack of unit access (SSH is not configured by default).

To Reproduce

juju version # 4.0.1-genericlinux-amd64
juju add-model test
juju deploy hello-juju
jhack fire hello-juju/0 update-status
hello-juju/0 update-status
╭───────────────────────────────────────────────────────────────────────────────────────────────────────── Traceback (most recent call last) ──────────────────────────────────────────────────────────────────────────────────────────────────────────╮
│ /snap/jhack/601/lib/python3.12/site-packages/jhack/utils/simulate_event.py:570 in simulate_event                                                                                                                                                     │
│                                                                                                                                                                                                                                                      │
│   567 │   Especially useful in combination with jhack charm sync and/or debug-code/debug-hooks ╭───────────── locals ──────────────╮                                                                                                                 │
│   568 │   """                                                                                  │      check_name = None            │                                                                                                                 │
│   569 │   print(target, event)                                                                 │         dry_run = None            │                                                                                                                 │
│ ❱ 570 │   return _simulate_event(                                                              │    env_override = None            │                                                                                                                 │
│   571 │   │   target,                                                                          │           event = 'update-status' │                                                                                                                 │
│   572 │   │   event,                                                                           │           model = None            │                                                                                                                 │
│   573 │   │   relation_remote=relation_remote,                                                 │       notice_id = None            │                                                                                                                 │
│                                                                                                │        parallel = None            │                                                                                                                 │
│                                                                                                │     relation_id = None            │                                                                                                                 │
│                                                                                                │ relation_remote = None            │                                                                                                                 │
│                                                                                                │          secret = None            │                                                                                                                 │
│                                                                                                │     show_output = True            │                                                                                                                 │
│                                                                                                │          target = 'hello-juju/0'  │                                                                                                                 │
│                                                                                                ╰───────────────────────────────────╯                                                                                                                 │
│                                                                                                                                                                                                                                                      │
│ /...

</details>



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- Fixes canonical/jhack#234

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