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/session.py

@@ -161,11 +161,18 @@ def _bind_access(
             ensure_swarm_comfy_installed(cfg, ip, log)
         except Exception as exc:
             log(f"SwarmUI Comfy install: {exc}")
+            try:
+                from gpu_rent.ready import collect_swarm_diagnostics
+
+                collect_swarm_diagnostics(cfg, ip, log)
+            except Exception as diag_exc:
+                log(f"diag: {diag_exc}")
             raise
         clock.mark("comfy-install", log)
         try:
             wait_backend_idle(cfg, ip, log)
         except CloudError as exc:
+            # wait_backend_idle already collected diag on errored/timeout
             log(f"ready: {exc}")
             raise
         clock.mark("Idle", log)