fix: honor explicit zero fee in scheduled bodies

Signed-off-by: exploreriii <133720349+exploreriii@users.noreply.github.com>

Author: exploreriii

src/hiero_sdk_python/transaction/transaction.py

@@ -513,8 +513,8 @@ def build_base_scheduled_body(self) -> SchedulableTransactionBody:
         """
         schedulable_body = SchedulableTransactionBody()
 
-        fee = self._transaction_fee or self._default_transaction_fee
-        if hasattr(fee, "to_tinybars"):
+        fee = self._transaction_fee if self._transaction_fee is not None else self._default_transaction_fee
+        if isinstance(fee, Hbar):
             schedulable_body.transactionFee = int(fee.to_tinybars())
         else:
             schedulable_body.transactionFee = int(fee)

tests/unit/transaction_freeze_and_bytes_test.py

@@ -15,6 +15,7 @@
 
 from hiero_sdk_python.account.account_id import AccountId
 from hiero_sdk_python.crypto.private_key import PrivateKey
+from hiero_sdk_python.hapi.services import transaction_pb2
 from hiero_sdk_python.hapi.services.transaction_response_pb2 import (
     TransactionResponse as TransactionResponseProto,
 )
@@ -795,6 +796,20 @@ def test_max_transaction_fee_survives_to_bytes_round_trip(mock_client):
     assert restored._transaction_fee == Hbar(2).to_tinybars()
 
 
+def test_explicit_zero_fee_is_honored_on_direct_and_scheduled_paths(mock_client):
+    """An explicit zero fee must reach both the direct and the scheduled body unchanged."""
+    tx = TransferTransaction()
+    tx.set_max_transaction_fee(Hbar(0))
+
+    scheduled_body = tx.build_base_scheduled_body()
+    assert scheduled_body.transactionFee == 0
+
+    tx.freeze_with(mock_client)
+    body = transaction_pb2.TransactionBody()
+    body.ParseFromString(next(iter(tx._transaction_body_bytes.values())))
+    assert body.transactionFee == 0
+
+
 def test_freeze_with_bare_magicmock_client_resolves_default_fee():
     """Freezing against a bare MagicMock client must not raise and must fall back to the per-type default fee."""
     tx = TransferTransaction()