Extract util for HTLCIntercepted event creation

We have an overarching goal of (mostly) getting rid of ChannelManager
persistence and rebuilding the ChannelManager's state from existing
ChannelMonitors, due to issues when the two structs are out-of-sync on restart.
The main issue that can arise is channel force closure.

As part of rebuilding ChannelManager forward HTLCs maps, we will also add
a fix that will regenerate HTLCIntercepted events for HTLC intercepts that
are present but have no corresponding event in the queue. That fix will use
this new method.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -3911,6 +3911,25 @@ macro_rules! process_events_body {
 	}
 }
 
+/// Creates an [`Event::HTLCIntercepted`] from a [`PendingAddHTLCInfo`]. We generate this event in a
+/// few places so this DRYs the code.
+fn create_htlc_intercepted_event(
+	intercept_id: InterceptId, pending_add: &PendingAddHTLCInfo,
+) -> Result<Event, ()> {
+	let inbound_amount_msat = pending_add.forward_info.incoming_amt_msat.ok_or(())?;
+	let requested_next_hop_scid = match pending_add.forward_info.routing {
+		PendingHTLCRouting::Forward { short_channel_id, .. } => short_channel_id,
+		_ => return Err(()),
+	};
+	Ok(Event::HTLCIntercepted {
+		requested_next_hop_scid,
+		payment_hash: pending_add.forward_info.payment_hash,
+		inbound_amount_msat,
+		expected_outbound_amount_msat: pending_add.forward_info.outgoing_amt_msat,
+		intercept_id,
+	})
+}
+
 impl<
 		M: Deref,
 		T: Deref,
@@ -11486,22 +11505,15 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 						let mut pending_intercepts = self.pending_intercepted_htlcs.lock().unwrap();
 						match pending_intercepts.entry(intercept_id) {
 							hash_map::Entry::Vacant(entry) => {
-								new_intercept_events.push_back((
-									events::Event::HTLCIntercepted {
-										requested_next_hop_scid: scid,
-										payment_hash,
-										inbound_amount_msat: pending_add
-											.forward_info
-											.incoming_amt_msat
-											.unwrap(),
-										expected_outbound_amount_msat: pending_add
-											.forward_info
-											.outgoing_amt_msat,
-										intercept_id,
-									},
-									None,
-								));
-								entry.insert(pending_add);
+								if let Ok(intercept_ev) =
+									create_htlc_intercepted_event(intercept_id, &pending_add)
+								{
+									new_intercept_events.push_back((intercept_ev, None));
+									entry.insert(pending_add);
+								} else {
+									debug_assert!(false);
+									fail_intercepted_htlc(pending_add);
+								}
 							},
 							hash_map::Entry::Occupied(_) => {
 								log_info!(