f - inline fresh selection in stfu_after_splice_locked test

The earlier fixup dropped do_initiate_rbf_splice_in's value_added
parameter and switched it to amend semantics. This test specifically
verifies that DiscardFunding fires on the can_initiate_rbf failure
path with a unique input surviving filtering, which requires fresh
input selection across rounds. Inline the splice_channel +
funding_contributed flow with without_prior_contribution rather than
using the helper to preserve the original assertions.

Author: jkczyz

lightning/src/ln/splicing_tests.rs

@@ -5196,10 +5196,21 @@ fn test_splice_rbf_stfu_after_splice_locked() {
 	// Provide more UTXOs for the RBF attempt.
 	provide_utxo_reserves(&nodes, 2, added_value * 2);
 
-	// Initiate RBF from node 0.
+	// Initiate RBF from node 0 with fresh inputs so the RBF round has a unique input that
+	// survives filtering when the failure cleanup runs.
 	let rbf_feerate = FeeRate::from_sat_per_kwu(FEERATE_FLOOR_SATS_PER_KW as u64 + 25);
-	let _funding_contribution =
-		do_initiate_rbf_splice_in(&nodes[0], &nodes[1], channel_id, added_value, rbf_feerate);
+	let funding_template = nodes[0].node.splice_channel(&channel_id, &node_id_1).unwrap();
+	let wallet = WalletSync::new(Arc::clone(&nodes[0].wallet_source), nodes[0].logger);
+	let funding_contribution = funding_template
+		.without_prior_contribution(rbf_feerate, FeeRate::MAX)
+		.with_coin_selection_source_sync(&wallet)
+		.add_value(added_value)
+		.build()
+		.unwrap();
+	nodes[0]
+		.node
+		.funding_contributed(&channel_id, &node_id_1, funding_contribution.clone(), None)
+		.unwrap();
 
 	// Node 0 sends STFU (can_initiate_rbf passes since no splice_locked yet).
 	let stfu_init = get_event_msg!(nodes[0], MessageSendEvent::SendStfu, node_id_1);