Expose fallback tx off sender session history

Extract the unchecked transaction from the original psbt which
becomes asccessible in the first session event -- withReplyKey.

Author: arminsabouri

payjoin-ffi/src/send/uni.rs

@@ -73,6 +73,11 @@ impl SenderSessionHistory {
     pub fn endpoints(&self) -> Option<Arc<Url>> {
         self.0.0.endpoint().map(|url| Arc::new(url.clone().into()))
     }
+
+    /// Fallback transaction from the session if present
+    pub fn fallback_tx(&self) -> Option<Arc<crate::Transaction>> {
+        self.0.0.fallback_tx().map(|tx| Arc::new(tx.into()))
+    }
 }
 
 #[derive(uniffi::Object)]

payjoin/src/send/v2/session.rs

@@ -67,6 +67,15 @@ pub struct SessionHistory {
 }
 
 impl SessionHistory {
+    /// Fallback transaction from the session if present
+    pub fn fallback_tx(&self) -> Option<bitcoin::Transaction> {
+        self.events.iter().find_map(|event| match event {
+            SessionEvent::CreatedReplyKey(proposal) =>
+                Some(proposal.v1.psbt.clone().extract_tx_unchecked_fee_rate()),
+            _ => None,
+        })
+    }
+
     pub fn endpoint(&self) -> Option<&Url> {
         self.events.iter().find_map(|event| match event {
             SessionEvent::V2GetContext(ctx) => Some(&ctx.endpoint),