Enhance backend recovery and diagnostics in installation and tuning scripts

- Improved the `install_swarm_comfy` function to handle empty backend states more effectively, introducing recovery mechanisms and enhanced logging for better visibility.
- Updated the `tune_swarm_perf` function to always sanitize backend FDS corruption, ensuring consistent performance tuning.
- Added new tests to validate the functionality of backend recovery and FDS sanitization, ensuring robustness in backend management.

src/gpu_rent/remote/install_swarm_comfy.py

@@ -657,11 +657,21 @@ def main() -> int:
         return 0
 
     if comfy_venv_ok() and bstat == "empty":
-        if installed is not True:
-            print("venv есть, IsInstalled=false — запускаю InstallConfirmWS")
+        print(
+            "backend empty при venv/IsInstalled — recover (sanitize FDS / AddNewBackend)…",
+            flush=True,
+        )
+        bstat = recover_empty_backends()
+        if bstat not in ("empty", "unknown") and not bstat.startswith("error:"):
+            print(f"recovered empty → {bstat}")
+            return 0
+        if installed is True:
+            print(
+                "WARN: всё ещё empty после recover — пробую InstallConfirmWS",
+                flush=True,
+            )
         else:
-            run_diagnostics()
-            return 1
+            print("venv есть, IsInstalled=false — запускаю InstallConfirmWS")
 
     marker = DATA / ".gpu-rent-comfy-installing"
     try: