Fix session state poisoning panic, harden session state access

handle_session_ack() had a bug where take_state() followed by
a non-Initiating match would re-insert the entry with state=None,
causing panics on subsequent .state()/.state_mut() calls. Fix by
checking is_initiating() before take_state().

Add safe is_initiating()/is_responding() to SessionEntry (matching
existing is_established()) and convert all production .state() callers
to use these None-safe methods. Gate .state() with #[cfg(test)].

src/node/session.rs

@@ -112,6 +112,7 @@ impl SessionEntry {
     }
 
     /// Get the current session state.
+    #[cfg(test)]
     pub(crate) fn state(&self) -> &EndToEndState {
         self.state.as_ref().expect("session state taken but not restored")
     }
@@ -147,6 +148,16 @@ impl SessionEntry {
         self.state.as_ref().is_some_and(|s| s.is_established())
     }
 
+    /// Check if we are the initiator (waiting for ack).
+    pub(crate) fn is_initiating(&self) -> bool {
+        self.state.as_ref().is_some_and(|s| s.is_initiating())
+    }
+
+    /// Check if we are the responder (sent ack, waiting for data).
+    pub(crate) fn is_responding(&self) -> bool {
+        self.state.as_ref().is_some_and(|s| s.is_responding())
+    }
+
     /// Get creation time.
     #[cfg(test)]
     pub(crate) fn created_at(&self) -> u64 {