[Hybrid] calling get_mamba_groups() once at MambaCopyBuffers.create() (#37318)

Signed-off-by: Francesco Fusco <ffu@zurich.ibm.com>

tests/v1/worker/test_mamba_utils.py

@@ -36,6 +36,7 @@ def test_resumed_req_ids_cleared_from_mamba_state_idx():
     spec = MagicMock(block_size=64, num_speculative_blocks=0)
     cache_config = MagicMock(enable_prefix_caching=True)
     input_batch = MagicMock(req_ids=[])
+    copy_bufs = MagicMock(mamba_group_ids=[0], mamba_spec=spec)
 
     mamba_state_idx = {
         "finished": 1,
@@ -62,7 +63,7 @@ def test_resumed_req_ids_cleared_from_mamba_state_idx():
             {},
             {},
             (),
-            MagicMock(),
+            copy_bufs,
         )
 
     assert mamba_state_idx == {"keep": 99}