[refactor] replace scattered business kwargs with typed request objects and explicit stage boundaries (#7024)

### What this PR does / why we need it?
Refactor `vllm_ascend/ops/fused_moe` to replace scattered MoE business
`**kwargs` with typed request objects and explicit stage boundaries.

- Prepare, dispatch, MLP, and quant stages now have clearer ownership.
- Main MoE path no longer depends on business `kwargs.get(...)` lookups.
- Comm and dispatcher interfaces are request-only on the main path.
- UTs can assert stage-level fields directly instead of inferring
behavior indirectly.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
CI passed.

---------

Signed-off-by: linfeng-yuan <1102311262@qq.com>

tests/e2e/multicard/2-cards/test_offline_inference_distributed.py

@@ -245,6 +245,7 @@ def test_qwen3_dense_prefetch_mlp_weight_tp2(model):
 @patch.dict(os.environ, {"VLLM_ASCEND_ENABLE_FLASHCOMM1": "1"})
 @patch.dict(os.environ, {"ASCEND_AGGREGATE_ENABLE": "1"})
 @patch.dict(os.environ, {"HCCL_BUFFSIZE": "1024"})
+@wait_until_npu_memory_free()
 def test_deepseek3_2_w8a8_pruning_mtp_tp2_ep():
     short_example_prompts = [
         "Hello ",
@@ -272,6 +273,7 @@ def test_deepseek3_2_w8a8_pruning_mtp_tp2_ep():
 @patch.dict(os.environ, {"VLLM_ASCEND_ENABLE_FLASHCOMM1": "1"})
 @patch.dict(os.environ, {"ASCEND_AGGREGATE_ENABLE": "1"})
 @patch.dict(os.environ, {"HCCL_BUFFSIZE": "1024"})
+@wait_until_npu_memory_free()
 def test_deepseek3_2_w8a8c8_pruning_mtp_tp2_ep():
     short_example_prompts = [
         "Hello ",