test: clarify mocked input-fee comment in trusted-mint test

The comment described "21 proofs @ 100 ppk" arithmetic, but the mocked
token has one proof and get_fees_for_proofs is hard-mocked to 3, so the
math wasn't exercised. Describe what the mock actually does.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_wallet.py

@@ -90,7 +90,7 @@ async def test_recieve_token_trusted_mint_deducts_input_fee() -> None:
 
     mock_wallet = Mock()
     mock_wallet.split = AsyncMock()
-    # 21 proofs @ 100 ppk -> (21*100 + 999) // 1000 = 3 sat input fee
+    # Mock a 3-sat input fee from the Cashu wallet API.
     mock_wallet.get_fees_for_proofs = Mock(return_value=3)
 
     from routstr.core.settings import settings