fix: encapsulate CancelledAt property and update unit tests for transaction cancellation

Assisted-by: GPT-5.4

Author: shokoban

src/SIPSorcery/core/SIPTransactions/SIPTransaction.cs

@@ -142,7 +142,7 @@ public string TransactionId
         /// For cancelled INVITEs this is the time they entered the cancelled state and is used to
         /// calculate expiry after T6. If unset, Created is used as a fallback by the transaction engine.
         /// </summary>
-        public DateTime CancelledAt;
+        public DateTime CancelledAt { get; private set; }
 
         protected string m_branchId;
         public string BranchId

test/unit/core/SIPTransactions/SIPTransactionEngineUnitTest.cs

@@ -233,14 +233,13 @@ public void CancelledInviteWithoutCancelledAt_ExpiresUsingCreatedFallback()
             SIPTransactionEngine engine = sipTransport.m_transactionEngine;
 
             SIPRequest inviteRequest = GetDummyINVITERequest(SIPURI.ParseSIPURI("sip:dummy@127.0.0.1:12014"));
-            var tx = new UACInviteTransaction(sipTransport, inviteRequest, null);
-            engine.AddTransaction(tx);
-
-            tx.Created = DateTime.Now.AddMilliseconds(-(SIPTimings.T6 + 1000));
-            tx.CancelledAt = DateTime.MinValue;
-
-            var stateField = typeof(SIPTransaction).GetField("m_transactionState", BindingFlags.Instance | BindingFlags.NonPublic);
-            stateField.SetValue(tx, SIPTransactionStatesEnum.Cancelled);
+            var tx = new UACInviteTransaction(sipTransport, inviteRequest, null);
+            engine.AddTransaction(tx);
+
+            tx.Created = DateTime.Now.AddMilliseconds(-(SIPTimings.T6 + 1000));
+
+            var stateField = typeof(SIPTransaction).GetField("m_transactionState", BindingFlags.Instance | BindingFlags.NonPublic);
+            stateField.SetValue(tx, SIPTransactionStatesEnum.Cancelled);
 
             var removeExpiredMethod = typeof(SIPTransactionEngine).GetMethod("RemoveExpiredTransactions", BindingFlags.Instance | BindingFlags.NonPublic);
             removeExpiredMethod.Invoke(engine, null);
@@ -260,17 +259,14 @@ public void CancelledInviteWithRecentCancelledAt_IsNotExpired()
             SIPTransactionEngine engine = sipTransport.m_transactionEngine;
 
             SIPRequest inviteRequest = GetDummyINVITERequest(SIPURI.ParseSIPURI("sip:dummy@127.0.0.1:12014"));
-            var tx = new UACInviteTransaction(sipTransport, inviteRequest, null);
-            engine.AddTransaction(tx);
-
-            tx.Created = DateTime.Now.AddMilliseconds(-(SIPTimings.T6 * 2));
-            tx.CancelledAt = DateTime.Now;
-
-            var stateField = typeof(SIPTransaction).GetField("m_transactionState", BindingFlags.Instance | BindingFlags.NonPublic);
-            stateField.SetValue(tx, SIPTransactionStatesEnum.Cancelled);
-
-            var removeExpiredMethod = typeof(SIPTransactionEngine).GetMethod("RemoveExpiredTransactions", BindingFlags.Instance | BindingFlags.NonPublic);
-            removeExpiredMethod.Invoke(engine, null);
+            var tx = new UACInviteTransaction(sipTransport, inviteRequest, null);
+            engine.AddTransaction(tx);
+
+            tx.Created = DateTime.Now.AddMilliseconds(-(SIPTimings.T6 * 2));
+            tx.CancelCall();
+
+            var removeExpiredMethod = typeof(SIPTransactionEngine).GetMethod("RemoveExpiredTransactions", BindingFlags.Instance | BindingFlags.NonPublic);
+            removeExpiredMethod.Invoke(engine, null);
 
             Assert.NotNull(engine.GetTransaction(inviteRequest));