Add diagnostics command and enhance error handling for backend states

- Introduced a new CLI command `diag` to collect diagnostics from SwarmUI/Comfy, including API, journal, and paths.
- Enhanced the `ensure_swarm_comfy_installed` function to include diagnostic script handling for errored backends.
- Updated `wait_backend_idle` to trigger diagnostics when backends are in an errored state, improving error recovery.
- Implemented fallback mechanisms for diagnostics in the `run_diagnostics` function, ensuring better visibility into backend issues.
- Added tests to validate the new diagnostic functionality and error handling, ensuring robustness in backend management.

src/gpu_rent/provision.py

@@ -93,8 +93,10 @@ def tune_swarm_perf(cfg: Config, host: str, log: Log) -> bool:
 
 
 def ensure_swarm_comfy_installed(cfg: Config, host: str, log: Log) -> None:
-    """Headless SwarmUI InstallConfirmWS (ComfyUI) when backends are still empty."""
-    log("SwarmUI first-install: ComfyUI backend (если ещё empty)…")
+    """Headless Comfy install / recover errored backends before ready wait."""
+    # Diag script next to install so recover-fail can subprocess it on the VM.
+    put_text(cfg, host, "/tmp/gpu-rent-swarm_diag.py", _pkg_text("swarm_diag.py"))
+    log("SwarmUI Comfy: install если empty, RestartBackends если errored…")
     run_python(
         cfg,
         host,