[Bugfix] fix bug for mtp (#6514)

### What this PR does / why we need it?
fix(mtp): resolve MTP core bugs and enhance eager mode test cases
1. Resolved critical issues in eager mode MTP core execution logic;
2. Fixed functional bugs in the _update_states_after_model_execute
function;
3. Updated and released test_mtp_qwen3_next.py to validate eager mode
acceptance rate.
### Does this PR introduce _any_ user-facing change?
None
### How was this patch tested?

- vLLM version: v0.15.0
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.15.0

Signed-off-by: Bowen-Leee <caoshankuangren@gmail.com>

vllm_ascend/worker/model_runner_v1.py

@@ -1374,6 +1374,9 @@ class NPUModelRunner(GPUModelRunner):
         with record_function_or_nullcontext("sample_token"):
             sampler_output = self._sample(logits, spec_decode_metadata)
 
+        if self.need_accepted_tokens:
+            self._update_states_after_model_execute(sampler_output.sampled_token_ids, scheduler_output)
+
         def propose_draft_token_ids(sampled_token_ids):
             assert spec_decode_common_attn_metadata is not None
             self._draft_token_ids = self.propose_draft_token_ids(
@@ -1474,8 +1477,6 @@ class NPUModelRunner(GPUModelRunner):
             logits,
             sampling_metadata,
         )
-        if self.need_accepted_tokens:  # TODO remove this if
-            self._update_states_after_model_execute(sampler_output.sampled_token_ids)
         return sampler_output
 
     # TODO: remove this func after eagle_proposer is refactored and