Fail request_refund_payment for unsupported chain

If a Refund has an unsupported chain, ChannelManager should not send an
invoice as it can't be paid on that chain. Instead, return an error when
calling ChannelManager::request_refund_payment for such refunds.

lightning/src/ln/channelmanager.rs

@@ -7895,6 +7895,10 @@ where
 		let amount_msats = refund.amount_msats();
 		let relative_expiry = DEFAULT_RELATIVE_EXPIRY.as_secs() as u32;
 
+		if refund.chain() != self.chain_hash {
+			return Err(Bolt12SemanticError::UnsupportedChain);
+		}
+
 		match self.create_inbound_payment(Some(amount_msats), relative_expiry, None) {
 			Ok((payment_hash, payment_secret)) => {
 				let payment_paths = self.create_blinded_payment_paths(amount_msats, payment_secret)

lightning/src/ln/offers_tests.rs

@@ -694,6 +694,36 @@ fn fails_creating_invoice_request_for_unsupported_chain() {
 	}
 }
 
+/// Fails requesting a payment when the refund contains an unsupported chain.
+#[test]
+fn fails_sending_invoice_with_unsupported_chain_for_refund() {
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 10_000_000, 1_000_000_000);
+
+	let alice = &nodes[0];
+	let bob = &nodes[1];
+
+	let absolute_expiry = Duration::from_secs(u64::MAX);
+	let payment_id = PaymentId([1; 32]);
+	let refund = bob.node
+		.create_refund_builder(
+			"refund".to_string(), 10_000_000, absolute_expiry, payment_id, Retry::Attempts(0), None
+		)
+		.unwrap()
+		.clear_chain()
+		.chain(Network::Signet)
+		.build().unwrap();
+
+	match alice.node.request_refund_payment(&refund) {
+		Ok(_) => panic!("Expected error"),
+		Err(e) => assert_eq!(e, Bolt12SemanticError::UnsupportedChain),
+	}
+}
+
 /// Fails creating an invoice request when a blinded reply path cannot be created without exposing
 /// the node's id.
 #[test]

lightning/src/offers/refund.rs

@@ -302,6 +302,11 @@ impl<'a, T: secp256k1::Signing> RefundBuilder<'a, T> {
 		self
 	}
 
+	pub(crate) fn clear_chain(mut self) -> Self {
+		self.refund.chain = None;
+		self
+	}
+
 	fn features_unchecked(mut self, features: InvoiceRequestFeatures) -> Self {
 		self.refund.features = features;
 		self