feat: add add_custom_fee to TopicUpdateTransaction (hiero-ledger#2566)

Signed-off-by: achintya2k5 <achintyasin@gmail.com>
Signed-off-by: Achintya Sinha <achintyasin@gmail.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Author: achintya2k5

src/hiero_sdk_python/consensus/topic_update_transaction.py

@@ -213,6 +213,27 @@ def set_fee_exempt_keys(self, keys: list[Key]) -> TopicUpdateTransaction:
         self.fee_exempt_keys = keys
         return self
 
+    def add_custom_fee(self, custom_fee: CustomFixedFee) -> TopicUpdateTransaction:
+        """
+        Adds a single custom fixed fee to the transaction's custom fee list.
+
+        Args:
+            custom_fee (CustomFixedFee): The custom fixed fee to add.
+
+        Returns:
+            TopicUpdateTransaction: The current instance for method chaining.
+        """
+
+        self._require_not_frozen()
+
+        if not isinstance(custom_fee, CustomFixedFee):
+            raise TypeError("custom_fee must be a CustomFixedFee")
+
+        if self.custom_fees is None:
+            self.custom_fees = []
+        self.custom_fees.append(custom_fee)
+        return self
+
     def clear_custom_fees(self) -> TopicUpdateTransaction:
         """
         Clears the custom fees for the topic update transaction and

tests/unit/topic_update_transaction_test.py

@@ -441,3 +441,48 @@ def test_auto_renew_period_omitted_when_unset(
     body = tx.build_transaction_body().consensusUpdateTopic
 
     assert not body.HasField("autoRenewPeriod")
+
+
+def test_add_custom_fee():
+    """Test adding a custom fee to the transaction."""
+    tx = TopicUpdateTransaction()
+    fee1 = CustomFixedFee(100, fee_collector_account_id=AccountId(0, 0, 123))
+    fee2 = CustomFixedFee(200, fee_collector_account_id=AccountId(0, 0, 456))
+
+    result = tx.add_custom_fee(fee1)
+
+    assert len(tx.custom_fees) == 1
+    assert tx.custom_fees[0] == fee1
+    assert result is tx
+
+    tx.add_custom_fee(fee2)
+
+    assert len(tx.custom_fees) == 2
+    assert tx.custom_fees[0] == fee1
+    assert tx.custom_fees[1] == fee2
+
+
+def test_add_custom_fee_frozen(mock_client, topic_id):
+    """Test calling add_custom_fee() after freezing raises an exception."""
+    tx = TopicUpdateTransaction()
+
+    tx.set_topic_id(topic_id)
+    tx.freeze_with(mock_client)
+
+    fee = CustomFixedFee(100, fee_collector_account_id=AccountId(0, 0, 123))
+
+    with pytest.raises(Exception, match="Transaction is immutable; it has been frozen"):
+        tx.add_custom_fee(fee)
+
+
+def test_add_custom_fee_type_error():
+    """Test passing None or a non-CustomFixedFee argument raises TypeError."""
+    tx = TopicUpdateTransaction()
+
+    with pytest.raises(TypeError, match="custom_fee must be a CustomFixedFee"):
+        tx.add_custom_fee("this_is_a_string")  # type: ignore
+    assert tx.custom_fees is None, "Invalid input must not change custom_fees"
+
+    with pytest.raises(TypeError, match="custom_fee must be a CustomFixedFee"):
+        tx.add_custom_fee(None)  # type: ignore
+    assert tx.custom_fees is None, "Invalid input must not change custom_fees"