[BugFix][MTP] Fix prefill misclassified as decode when prompt tokens == num_spec_tokens + 1 (#6835)

## Problem
When MTP is enabled, prefill requests with `prompt_tokens ==
num_spec_tokens + 1` are incorrectly classified as decode requests,
causing accuracy issues.

## Root Cause
The `uniform_decode` condition only checked:
- `max_num_scheduled_tokens == uniform_decode_query_len`
- `num_tokens == max_num_scheduled_tokens * num_reqs`

This is insufficient because a prefill request with specific prompt
length satisfies these conditions as well.

## Fix
Add `is_all_decode` check to ensure all requests have
`num_computed_tokens > 0` before classifying as uniform decode, since
decode requests must have computed at least one token.
- vLLM version: v0.15.0
- vLLM main:
83b47f67b1

---------

Signed-off-by: SlightwindSec <slightwindsec@gmail.com>

tests/e2e/multicard/4-cards/spec_decode/test_mtp_qwen3_next.py

@@ -100,6 +100,7 @@ def test_qwen3_next_mtp_correctness_tp4(model_name: str,
         "The president of the United States is",
         "The capital of France is",
         "The future of AI is",
+        "Who are you?",
     ]
 
     max_tokens = 20