[Test] add ut for decorator.py/deepseek_mtp.py (#2127)

### What this PR does / why we need it?
add ut for decorator.py/deepseek_mtp.py
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
CI passed with new tests
- vLLM version: v0.10.0
- vLLM main:
055bd3978e

---------

Signed-off-by: CaranLic <740821011@qq.com>

tests/ut/multistream/test_decorator.py

@@ -0,0 +1,47 @@
+import pytest
+from pytest_mock import MockFixture
+
+from tests.ut.base import PytestBase
+from vllm_ascend.multistream.decorator import set_multistream_support
+
+
+class Context:
+
+    def __init__(self, attn_metadata=None):
+        self.attn_metadata = attn_metadata
+
+
+class TestDecorator(PytestBase):
+
+    @pytest.mark.parametrize(
+        'layer_context, microbatch_context, expected_metadata', [
+            ((-1, None, None), -1, {
+                "original": True
+            }),
+            ((-1, None, None), 0, {
+                "original": True
+            }),
+            ((0, None, None), -1, {
+                "original": True
+            }),
+            ((0, None, [{
+                "new": True
+            }]), 0, {
+                "new": True
+            }),
+        ])
+    def test_decorator(self, mocker: MockFixture, layer_context,
+                       microbatch_context, expected_metadata):
+
+        def context_func():
+            return Context(attn_metadata={"original": True})
+
+        mocker.patch(
+            'vllm_ascend.multistream.decorator.get_multistream_layer_context',
+            return_value=layer_context)
+        mocker.patch(
+            'vllm_ascend.multistream.decorator.get_multistream_microbatch_context',
+            return_value=microbatch_context)
+
+        context = set_multistream_support()(context_func)()
+        assert context.attn_metadata == expected_metadata