Fix incorrect weight estimation for splice shared input

The splice shared input weight should always be composed of the base
input weight (prevout & sequence), an empty `script_sig` weight, and the
multisig channel-type specific witness weight.

Author: wpaulino

lightning/src/ln/interactivetxs.rs

@@ -1639,7 +1639,10 @@ impl InputOwned {
 	fn estimate_input_weight(&self) -> Weight {
 		match self {
 			InputOwned::Single(single) => estimate_input_weight(&single.prev_output),
-			InputOwned::Shared(shared) => estimate_input_weight(&shared.prev_output),
+			// TODO(taproot): Needs to consider different weights based on channel type
+			InputOwned::Shared(_) => Weight::from_wu(
+				BASE_INPUT_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT + FUNDING_TRANSACTION_WITNESS_WEIGHT,
+			),
 		}
 	}
 
@@ -2156,6 +2159,8 @@ pub(super) fn calculate_change_output_value(
 		weight = weight.saturating_add(TX_COMMON_FIELDS_WEIGHT);
 		if is_splice {
 			// TODO(taproot): Needs to consider different weights based on channel type
+			weight = weight.saturating_add(BASE_INPUT_WEIGHT);
+			weight = weight.saturating_add(EMPTY_SCRIPT_SIG_WEIGHT);
 			weight = weight.saturating_add(FUNDING_TRANSACTION_WITNESS_WEIGHT);
 		}
 	}
@@ -3147,7 +3152,7 @@ mod tests {
 		// Provide and expect a shared input
 		do_test_interactive_tx_constructor(TestSession {
 			description: "Provide and expect a shared input",
-			inputs_a: generate_inputs(&[TestOutput::P2WPKH(50_000)]),
+			inputs_a: generate_inputs(&[TestOutput::P2WPKH(100_000)]),
 			a_shared_input: Some(generate_shared_input(&prev_funding_tx_1, 0, 60_000)),
 			shared_output_a: generate_funding_txout(108_000, 108_000),
 			outputs_a: vec![],

lightning/src/ln/splicing_tests.rs

@@ -235,7 +235,7 @@ fn test_v1_splice_in() {
 		assert_eq!(tx_add_output_msg.sats, post_splice_channel_value);
 	} else {
 		assert!(tx_add_output_msg.script.is_p2wpkh());
-		assert_eq!(tx_add_output_msg.sats, 14146); // extra_splice_funding_input_sats - splice_in_sats
+		assert_eq!(tx_add_output_msg.sats, 13979); // extra_splice_funding_input_sats - splice_in_sats
 	}
 
 	let _res = acceptor_node