fix: clean up config.py unused imports, null-YAML safety, and add non-default roundtrip test

tests/unit/test_config.py

@@ -2,7 +2,7 @@ import pytest
 import click
 from autossh_mgr.config import (
     TunnelConfig, ensure_dirs, load_tunnels, save_tunnels, get_tunnel
-)
+)  # TunnelConfig kept for test_save_preserves_non_default_values
 
 
 def test_ensure_dirs_creates_structure(tmp_path):
@@ -59,3 +59,22 @@ def test_get_config_dir_env_override(monkeypatch, tmp_path):
     monkeypatch.setenv("AUTOSSH_MGR_CONFIG_DIR", str(tmp_path))
     from autossh_mgr.config import get_config_dir
     assert get_config_dir() == tmp_path
+
+
+def test_save_preserves_non_default_values(config_dir):
+    tunnel = TunnelConfig(
+        name="custom", host="h.com", user="u",
+        local_port=80, remote_port=8080,
+        port=2222,
+        identity_file="~/.ssh/custom_key",
+        local_host="0.0.0.0",
+        remote_host="127.0.0.1",
+        ssh_options="-v",
+    )
+    save_tunnels(config_dir, [tunnel])
+    loaded = load_tunnels(config_dir)[0]
+    assert loaded.port == 2222
+    assert loaded.identity_file == "~/.ssh/custom_key"
+    assert loaded.local_host == "0.0.0.0"
+    assert loaded.remote_host == "127.0.0.1"
+    assert loaded.ssh_options == "-v"