GRM-17: fix: make grm list retrieve runner status correctly

- run_ad_hoc() now raises AnsibleError on non-zero exit, surfacing
  stderr instead of silently returning empty stdout
- list_runners() passes become=True to run_ad_hoc since systemctl
  is-active requires root privileges
- Add i18n translations for ad-hoc failure messages
- Add unit test for run_ad_hoc failure case
- Update list_runners test to expect become=True
- 119 tests, 100% coverage, pyright clean, ruff clean

src/gitea_runner_manager/executor.py

@@ -109,6 +109,15 @@ class AnsibleExecutor:
             capture_output=True,
             text=True,
         )
+        if proc.returncode != 0:
+            stderr = proc.stderr.strip() if proc.stderr else ""
+            raise AnsibleError(
+                _(
+                    "Ad-hoc command failed on {host}: {stderr}",
+                    host=host,
+                    stderr=stderr or _("exit code {code}", code=proc.returncode),
+                )
+            )
         return proc.stdout.strip()
 
     def _extract_status(self, log_file: Path) -> str | None: