Introduce amount check in pay_for_offer

We introduce this check in pay_for_offer, to ensure that
if the offer amount is specified in currency, a corresponding amount
to be used in invoice request must be provided.

**Reasoning:** When responding to an offer with currency, we enforce
that the invoice request must always include an amount. This ensures we
never receive an invoice tied to a currency-denominated offer without
a corresponding request amount.

By moving currency conversion upfront into the invoice request creation
where the user can supply their own conversion logic — we avoid pushing
conversion concerns into invoice parsing. This significantly reduces
complexity during invoice verification.

Author: shaavan

lightning/src/ln/channelmanager.rs

@@ -97,7 +97,7 @@ use crate::offers::invoice_request::{
 	DefaultCurrencyConversion, InvoiceRequest, InvoiceRequestVerifiedFromOffer,
 };
 use crate::offers::nonce::Nonce;
-use crate::offers::offer::{Offer, OfferFromHrn};
+use crate::offers::offer::{Amount, Offer, OfferFromHrn};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::Refund;
 use crate::offers::static_invoice::StaticInvoice;
@@ -13081,6 +13081,13 @@ where
 		let entropy = &*self.entropy_source;
 		let nonce = Nonce::from_entropy_source(entropy);
 
+		// If the offer is for a specific currency, ensure the amount is provided.
+		if let Some(Amount::Currency { iso4217_code: _, amount: _ }) = offer.amount() {
+			if amount_msats.is_none() {
+				return Err(Bolt12SemanticError::MissingAmount);
+			}
+		}
+
 		let builder = self.flow.create_invoice_request_builder(
 			offer, nonce, payment_id,
 		)?;