test: add test for duplicate port constraint

l4d2web/tests/test_servers.py

@@ -83,6 +83,37 @@ def test_reassign_blueprint_anytime(user_client_with_blueprints) -> None:
     assert response.status_code == 200
 
 
+def test_create_server_duplicate_port(user_client_with_blueprints) -> None:
+    client, data = user_client_with_blueprints
+    
+    # Create the first server
+    response = client.post(
+        "/servers",
+        data={"name": "server-1", "port": "27015", "blueprint_id": str(data["blueprint_id"])},
+        headers={"X-CSRF-Token": "test-token"},
+    )
+    assert response.status_code == 302
+    
+    # Try to create a second server with the same port
+    response = client.post(
+        "/servers",
+        data={"name": "server-2", "port": "27015", "blueprint_id": str(data["blueprint_id"])},
+        headers={"X-CSRF-Token": "test-token"},
+    )
+    assert response.status_code == 409
+    assert b"port already in use" in response.data
+    
+    # Verify the second server was not created by checking how many servers exist
+    from sqlalchemy import select
+    from l4d2web.db import session_scope
+    from l4d2web.models import Server
+    
+    with session_scope() as session:
+        servers = session.scalars(select(Server)).all()
+        assert len(servers) == 1
+        assert servers[0].name == "server-1"
+
+
 def test_lifecycle_form_creates_queued_job(user_client_with_blueprints) -> None:
     client, data = user_client_with_blueprints
     create_response = client.post(