Return error for invalid onchain send fee rate

Previously, an invalid fee_rate_sat_per_vb (overflows) was silently
collapsed into None via .and_then(), causing the node to substitute
its default fee estimation without informing the user. Now the server
returns an InvalidRequestError and the CLI rejects invalid fee rates
before sending the request.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: benthecarman

ldk-server-cli/src/main.rs

@@ -634,6 +634,12 @@ async fn main() {
 			);
 		},
 		Commands::OnchainSend { address, amount, send_all, fee_rate_sat_per_vb } => {
+			if let Some(rate) = fee_rate_sat_per_vb {
+				// Mirrors FeeRate::from_sat_per_vb: sat/vb * 1000/4 = sat/kwu
+				if rate.checked_mul(1000 / 4).is_none() {
+					handle_error_msg(&format!("Invalid fee rate: {} sat/vB", rate));
+				}
+			}
 			let amount_sats = amount.map(|a| a.to_sat().unwrap_or_else(|e| handle_error_msg(&e)));
 			handle_response_result::<_, OnchainSendResponse>(
 				client

ldk-server/src/api/onchain_send.rs

@@ -29,7 +29,12 @@ pub(crate) fn handle_onchain_send_request(
 			)
 		})?;
 
-	let fee_rate = request.fee_rate_sat_per_vb.and_then(FeeRate::from_sat_per_vb);
+	let fee_rate = match request.fee_rate_sat_per_vb {
+		Some(rate) => Some(FeeRate::from_sat_per_vb(rate).ok_or_else(|| {
+			LdkServerError::new(InvalidRequestError, format!("Invalid fee rate: {} sat/vB", rate))
+		})?),
+		None => None,
+	};
 	let txid = match (request.amount_sats, request.send_all) {
 		(Some(amount_sats), None) => {
 			context.node.onchain_payment().send_to_address(&address, amount_sats, fee_rate)?