Fix Decimal serialization in subTest for pytest-xdist

The parallel test execution failed on 4 tests because pytest-xdist's
execnet cannot serialize Decimal objects when sending test results
between workers.

Solution: Convert Decimal objects to strings in subTest() parameters.
The Decimal values are still used for actual test logic, but the
subTest labels now use string representations.

Fixed tests:
- test_sweep_restocking_fee_fee_amount_calculation
- test_doge_expected_amount_precision_always_rounds_up
- test_xmr_expected_amount_precision_always_rounds_up
- test_fee_calculations_always_round_up

This is a common pytest-xdist limitation with non-serializable types.

make_post_sell/tests/test_crypto_watcher.py

@@ -2849,7 +2849,7 @@ class SweepRestockingFeeTests(unittest.TestCase):
         ]
 
         for fee_amount, coin_type, expected_atomic in test_cases:
-            with self.subTest(fee_amount=fee_amount, coin_type=coin_type):
+            with self.subTest(fee_amount=str(fee_amount), coin_type=coin_type):
                 payment = MagicMock()
                 payment.coin_type = coin_type
                 payment.shop_sweep_to_address = None  # Will skip actual sweep
@@ -4913,7 +4913,7 @@ class CryptoPrecisionTests(unittest.TestCase):
         ]
 
         for amount in test_cases:
-            with self.subTest(amount=amount):
+            with self.subTest(amount=str(amount)):
                 # This is how crypto.py calculates expected amounts
                 expected_koinu = math.ceil(amount * 100_000_000)
 
@@ -4937,7 +4937,7 @@ class CryptoPrecisionTests(unittest.TestCase):
         ]
 
         for amount in test_cases:
-            with self.subTest(amount=amount):
+            with self.subTest(amount=str(amount)):
                 # This is how crypto.py calculates expected amounts
                 expected_piconero = math.ceil(amount * 1_000_000_000_000)
 
@@ -4992,7 +4992,7 @@ class CryptoPrecisionTests(unittest.TestCase):
         ]
 
         for fee in test_fees:
-            with self.subTest(fee=fee):
+            with self.subTest(fee=str(fee)):
                 # DOGE fee calculation
                 doge_fee_koinu = math.ceil(fee * 100_000_000)
                 naive_doge_fee = int(fee * 100_000_000)