[CI] Fix unreachable FakeReq field initialization (#37182)

test/registered/unit/mem_cache/test_streaming_session_unit.py

@@ -83,17 +83,37 @@ class _FakeReq:
         self.skip_lock_node_ids = {}
         self.mamba_pool_idx = None
         self.mamba_ping_pong_track_buffer = None
-
-    def detach_kv(self):
-        kv, self.kv = self.kv, ReqKvInfo()
-        return kv
         self.mamba_next_track_idx = None
+        self.mamba_last_track_idx = None
         self.mamba_last_track_seqlen = None
         self.mamba_branching_seqlen = None
         self.to_finish = None
         self.finished_reason = None
         self.finished_len = None
 
+    def detach_kv(self):
+        kv, self.kv = self.kv, ReqKvInfo()
+        return kv
+
+
+def test_session_slot_round_trip_preserves_mamba_state():
+    req = _FakeReq("session-a", req_pool_idx=0, committed=4, allocated=4)
+    req.mamba_next_track_idx = 1
+    req.mamba_last_track_idx = 0
+    req.mamba_last_track_seqlen = 3
+    req.mamba_branching_seqlen = 2
+
+    slot = SessionSlot()
+    slot.save_from_req(req, is_first=True)
+
+    next_req = _FakeReq("session-a", req_pool_idx=1, committed=0, allocated=0)
+    slot.restore_to_req(next_req)
+
+    assert next_req.mamba_next_track_idx == 1
+    assert next_req.mamba_last_track_idx == 0
+    assert next_req.mamba_last_track_seqlen == 3
+    assert next_req.mamba_branching_seqlen == 2
+
 
 def test_preabort_detaches_session_and_preserves_slot():
     """Pre-aborted req (to_finish set before match_prefix) is detached from