fix(esplora): use saturating_add in update_tx_graph()

This fixes overflow error when calling update_tx_graph() from update_tx_graph_without_keychain().

crates/esplora/src/async_ext.rs

@@ -261,7 +261,7 @@ impl EsploraAsyncExt for esplora_client::AsyncClient {
                     }
                 }
 
-                if last_index > last_active_index.map(|i| i + stop_gap as u32) {
+                if last_index > last_active_index.map(|i| i.saturating_add(stop_gap as u32)) {
                     break;
                 }
             }

crates/esplora/src/blocking_ext.rs

@@ -252,7 +252,7 @@ impl EsploraExt for esplora_client::BlockingClient {
                     }
                 }
 
-                if last_index > last_active_index.map(|i| i + stop_gap as u32) {
+                if last_index > last_active_index.map(|i| i.saturating_add(stop_gap as u32)) {
                     break;
                 }
             }