feat: use Amount on calculate_fee, fee_absolute, fee_amount and others

- update to use `bitcoin::Amount` on `CreateTxError::FeeTooLow` variant.
- update to use `bitcoin::Amount` on `Wallet::calculate_fee()`.
- update to use `bitcoin::Amount` on `FeePolicy::fee_absolute()`.
- update to use `bitcoin::SignedAmount` on
  `CalculateFeeError::NegativeFee` variant.
- update to use `bitcoin::Amount` on `TxGraph::calculate_fee()`.
- update to use `bitcoin::Amount` on `PsbUtils::fee_amount()`

crates/esplora/tests/async_ext.rs

@@ -92,7 +92,8 @@ pub async fn test_update_tx_graph_without_keychain() -> anyhow::Result<()> {
             .fee
             .expect("Fee must exist")
             .abs()
-            .to_sat() as u64;
+            .to_unsigned()
+            .expect("valid `Amount`");
 
         // Check that the calculated fee matches the fee from the transaction data.
         assert_eq!(fee, tx_fee);

crates/esplora/tests/blocking_ext.rs

@@ -92,7 +92,8 @@ pub fn test_update_tx_graph_without_keychain() -> anyhow::Result<()> {
             .fee
             .expect("Fee must exist")
             .abs()
-            .to_sat() as u64;
+            .to_unsigned()
+            .expect("valid `Amount`");
 
         // Check that the calculated fee matches the fee from the transaction data.
         assert_eq!(fee, tx_fee);