Reject quantity on variable-amount offers

The variable-amount path silently dropped quantity, so callers got an
offer that didn't match what they asked for. Return an invalid request
error instead.

Author: benthecarman

e2e-tests/tests/e2e.rs

@@ -336,6 +336,18 @@ async fn test_cli_bolt12_receive() {
 	let offer: Offer = offer_str.parse().unwrap();
 	let offer_id = <[u8; 32]>::from_hex(output["offer_id"].as_str().unwrap()).unwrap();
 	assert_eq!(offer.id().0, offer_id);
+
+	let error = server_a
+		.client()
+		.bolt12_receive(Bolt12ReceiveRequest {
+			description: "variable amount".to_string(),
+			amount_msat: None,
+			expiry_secs: None,
+			quantity: Some(3),
+		})
+		.await
+		.unwrap_err();
+	assert_eq!(error.error_code, InvalidRequestError);
 }
 
 #[tokio::test]

ldk-server/src/api/bolt12_receive.rs

@@ -13,6 +13,7 @@ use hex::DisplayHex;
 use ldk_server_grpc::api::{Bolt12ReceiveRequest, Bolt12ReceiveResponse};
 
 use crate::api::error::LdkServerError;
+use crate::api::error::LdkServerErrorCode::InvalidRequestError;
 use crate::service::Context;
 
 pub(crate) async fn handle_bolt12_receive_request(
@@ -25,6 +26,12 @@ pub(crate) async fn handle_bolt12_receive_request(
 			request.expiry_secs,
 			request.quantity,
 		)?,
+		None if request.quantity.is_some() => {
+			return Err(LdkServerError::new(
+				InvalidRequestError,
+				"quantity can only be set for fixed-amount offers".to_string(),
+			));
+		},
 		None => context
 			.node
 			.bolt12_payment()