[Feat][SP] Suport SP for VL MoE models (#7044)

### What this PR does / why we need it?

2nd PR for https://github.com/vllm-project/vllm-ascend/issues/5712,
extend SP to VL MoE models.


### Does this PR introduce _any_ user-facing change?
remove `sp_threshold` in additional config and reuse `sp_min_token_num`
from vLLM.


### How was this patch tested?
- Model: Qwen3-VL-30B-A3B, 
- TP4 DP2
- 100 reqs
- max concurrency 1

| Seq length | Mean TTFT (ms) main | Mean TTFT (ms) this PR |
|------------|---------------------|------------------------|
| 4k         | 429.40               | 323.3                  |
| 16k        | 1297.01              | 911.74                |

- vLLM version: v0.16.0
- vLLM main:
4034c3d32e

---------

Signed-off-by: realliujiaxu <realliujiaxu@163.com>

tests/ut/ops/test_prepare_finalize.py

@@ -188,7 +188,10 @@ class TestPrepareAndFinalize(unittest.TestCase):
     @patch('vllm_ascend.ascend_forward_context.get_forward_context')
     @patch("vllm_ascend.ops.fused_moe.prepare_finalize.enable_sp",
            return_value=False)
-    def test_allgather_prepare_finalize(self, mock_enable_sp,
+    @patch("vllm_ascend.ops.fused_moe.prepare_finalize.enable_sp_by_pass",
+        return_value=False)
+    def test_allgather_prepare_finalize(self, mock_enable_sp_by_pass,
+                                        mock_enable_sp,
                                         mock_get_forward_context,
                                         mock_get_dp_group):
         # Mock forward context