[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>

.github/workflows/scripts/config.yaml

@@ -152,4 +152,3 @@ e2e-multicard-4-cards:
     is_skipped: true
   - name: tests/e2e/multicard/4-cards/spec_decode/test_mtp_qwen3_next.py
     estimated_time: 60
-    is_skipped: true