[Nemotron] Fix decode track-save reading the stale tail of the CUDA-graph track buffer (#32555)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

python/sglang/srt/layers/attention/hybrid_linear_attn_backend.py

@@ -566,12 +566,15 @@ class MambaAttnBackendBase(AttentionBackend):
             self.state_indices_list[bs - 1][: len(mamba_indices)].copy_(mamba_indices)
         # Refresh the static track-dest buffer in-place (translated); the captured
         # track-save reads it, leaving the handed-in InputBuffer slot read-only.
+        # Hand out only the refreshed [:bs] prefix — Mamba2's track-save slices
+        # [-num_decodes:], which on the full max_bs buffer binds the stale tail.
         track_buf = None
         if mamba_track_indices is not None:
-            track_buf = self.mamba_track_indices_buf
-            track_buf[: len(mamba_track_indices)].copy_(
-                self._translate_mamba_indices(mamba_track_indices)
-            )
+            assert (
+                len(mamba_track_indices) >= bs
+            ), f"{len(mamba_track_indices)=} < {bs=}"
+            track_buf = self.mamba_track_indices_buf[:bs]
+            track_buf.copy_(self._translate_mamba_indices(mamba_track_indices[:bs]))
         # Refresh the static write cursor in-place (mirrors the eager
         # snapshot-then-advance). Skip the advance during capture: dummy slots
         # would corrupt real ring positions.