fix: correct PermissionError handling in is_process_alive, add corrupt PID test

src/autossh_mgr/process.py

@@ -47,8 +47,10 @@ def is_process_alive(pid: int) -> bool:
     try:
         os.kill(pid, 0)
         return True
-    except (ProcessLookupError, PermissionError):
+    except ProcessLookupError:
         return False
+    except PermissionError:
+        return True  # process exists but owned by another user
 
 
 def check_stale(config_dir: Path, name: str) -> bool: