feat(ssh): support full multi-hop jump chain (#356) (#1058)

* feat(ssh): support full multi-hop jump chain (#356)

* fix(edit): validate jump cycle for new server saves

test/jump_chain_test.dart

@@ -0,0 +1,91 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:server_box/core/utils/jump_chain.dart';
+import 'package:server_box/data/model/server/server_private_info.dart';
+
+Spi _spi({required String id, required String name, String? jumpId}) {
+  return Spi(
+    id: id,
+    name: name,
+    ip: '$name.example.com',
+    port: 22,
+    user: 'root',
+    jumpId: jumpId,
+  );
+}
+
+void main() {
+  group('JumpChain', () {
+    test('wouldCreateJumpCycle returns false for valid chain', () {
+      final servers = <String, Spi>{
+        'A': _spi(id: 'A', name: 'a'),
+        'B': _spi(id: 'B', name: 'b', jumpId: 'A'),
+      };
+
+      final result = wouldCreateJumpCycle(
+        currentServerId: 'C',
+        candidateJumpId: 'B',
+        serversById: servers,
+      );
+
+      expect(result, isFalse);
+    });
+
+    test('wouldCreateJumpCycle detects cycle back to current server', () {
+      final servers = <String, Spi>{
+        'A': _spi(id: 'A', name: 'a', jumpId: 'B'),
+        'B': _spi(id: 'B', name: 'b', jumpId: 'C'),
+        'C': _spi(id: 'C', name: 'c'),
+      };
+
+      final result = wouldCreateJumpCycle(
+        currentServerId: 'C',
+        candidateJumpId: 'A',
+        serversById: servers,
+      );
+
+      expect(result, isTrue);
+    });
+
+    test('wouldCreateJumpCycle treats existing malformed loop as invalid', () {
+      final servers = <String, Spi>{
+        'A': _spi(id: 'A', name: 'a', jumpId: 'B'),
+        'B': _spi(id: 'B', name: 'b', jumpId: 'A'),
+      };
+
+      final result = wouldCreateJumpCycle(
+        currentServerId: 'C',
+        candidateJumpId: 'A',
+        serversById: servers,
+      );
+
+      expect(result, isTrue);
+    });
+
+    test('wouldCreateJumpCycle validates new server with null current id', () {
+      final servers = <String, Spi>{
+        'A': _spi(id: 'A', name: 'a', jumpId: 'B'),
+        'B': _spi(id: 'B', name: 'b', jumpId: 'A'),
+      };
+
+      final result = wouldCreateJumpCycle(
+        currentServerId: null,
+        candidateJumpId: 'A',
+        serversById: servers,
+      );
+
+      expect(result, isTrue);
+    });
+
+    test('collectJumpServers collects reachable jump servers', () {
+      final target = _spi(id: 'T', name: 'target', jumpId: 'A');
+      final servers = <String, Spi>{
+        'A': _spi(id: 'A', name: 'a', jumpId: 'B'),
+        'B': _spi(id: 'B', name: 'b'),
+      };
+
+      final chain = collectJumpServers(spi: target, serversById: servers);
+
+      expect(chain.keys.toList(), ['A', 'B']);
+    });
+  });
+}