Implement idle-killer enhancements and swarm management improvements

- Marked critical bugs as resolved in the review documentation, including changes to the `arm_idle_killer` function to raise errors on credential creation failures and ensure proper file permissions for JSON credentials.
- Introduced a new `_try_arm_idle_killer` function in `provision.py` to manage idle-killer state more effectively, ensuring it arms correctly during provisioning.
- Updated the `swarm_busy` function in `remote/idle_killer.py` to allow idle state after a specified duration of Swarm unavailability, preventing unnecessary billing.
- Enhanced performance tuning logic in `tune_swarm_perf.py` to ensure proper handling of pip installation success before applying extra arguments.
- Added tests to validate the new idle-killer behavior and swarm management logic, ensuring robustness in handling idle states and error conditions.

src/gpu_rent/idle_killer.py

@@ -146,14 +146,15 @@ def arm_idle_killer(
     log: Log,
 ) -> None:
     if not server_id:
-        log("idle-killer: нет server_id — пропуск")
-        return
-    try:
-        creds = create_application_credential(conn, cfg, server_id, log)
-    except CloudError as exc:
-        log(f"idle-killer слеп: {exc}")
-        return
-    put_text(cfg, host, "/tmp/gpu-rent-idle-killer.json", json.dumps(creds, indent=2) + "\n")
+        raise CloudError("idle-killer: нет server_id")
+    creds = create_application_credential(conn, cfg, server_id, log)
+    put_text(
+        cfg,
+        host,
+        "/tmp/gpu-rent-idle-killer.json",
+        json.dumps(creds, indent=2) + "\n",
+        mode=0o600,
+    )
     run_ssh(
         cfg,
         host,