Session idle timeout now based on application data only

MMP reports (SenderReport, ReceiverReport, PathMtuNotification) were
resetting the idle timer on both RX and TX paths, preventing sessions
from ever timing out when MMP traffic kept flowing. Changed touch() to
only be called for DataPacket send/receive and session establishment,
so sessions with no application data tear down after the idle timeout
even with active MMP measurement traffic.

src/node/tests/session.rs

@@ -1307,6 +1307,36 @@ fn test_purge_idle_sessions_disabled_when_zero() {
     assert_eq!(node.session_count(), 1, "Sessions should not be purged when idle timeout is disabled");
 }
 
+#[test]
+fn test_purge_idle_sessions_mmp_activity_does_not_prevent_purge() {
+    let mut node = make_node();
+    let remote = Identity::generate();
+    let remote_addr = *remote.node_addr();
+
+    let session = make_noise_session(node.identity(), &remote);
+    let entry = crate::node::session::SessionEntry::new(
+        remote_addr,
+        remote.pubkey_full(),
+        EndToEndState::Established(session),
+        1000, // created at t=1s
+        true,
+    );
+
+    // Do NOT call entry.touch() — simulates a session where only MMP
+    // reports have flowed (MMP no longer calls touch). last_activity
+    // remains at creation time (1000ms).
+    node.sessions.insert(remote_addr, entry);
+
+    // Purge at t=92s — 91s since creation, exceeds 90s idle timeout.
+    // Even though MMP reports would have been flowing, they no longer
+    // reset the idle timer.
+    let now_ms = 92_000;
+    node.purge_idle_sessions(now_ms);
+
+    assert_eq!(node.session_count(), 0,
+        "Session with MMP-only activity should be purged");
+}
+
 // ============================================================================
 // Unit tests: COORDS_PRESENT warmup counter
 // ============================================================================