refactor : use factory for attention backends

- register built-in backends through BaseFactory
- derive benchmark choices from registered backends
- cover string selection and invalid backend names

tests/extension/test_backend.py

@@ -8,6 +8,7 @@ import pytest
 
 from astrai.extension import (
     ATTN_BACKEND,
+    AttentionBackendFactory,
     CudaBackend,
     TorchNativeBackend,
     attn_backend,
@@ -26,6 +27,26 @@ def test_attn_backend_context_with_enum():
     assert isinstance(get_backend(), TorchNativeBackend)
 
 
+def test_attn_backend_context_with_registered_name():
+    with attn_backend("cuda"):
+        assert isinstance(get_backend(), CudaBackend)
+    assert isinstance(get_backend(), TorchNativeBackend)
+
+
+def test_attention_backend_factory_lists_builtin_backends():
+    assert AttentionBackendFactory.list_registered() == [
+        "cuda",
+        "flash",
+        "torch_native",
+    ]
+
+
+def test_attn_backend_rejects_unknown_registered_name():
+    with pytest.raises(ValueError, match="Unknown component: 'unknown'"):
+        with attn_backend("unknown"):
+            pass
+
+
 def test_attn_backend_context_with_class():
     with attn_backend(CudaBackend):
         assert isinstance(get_backend(), CudaBackend)