[PD] bug fix: Update status if nixl receiver send a a dummy req.

[thesues]

@trevor-m
@jokerwyt

For PD, if Decode's tp-size is less than Prefill's tp-size. Prefill will throw this exception.

Exception in thread Thread-5 (bootstrap_thread):
Traceback (most recent call last):
  File "/usr/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
Exception in thread Thread-5 (bootstrap_thread):
Traceback (most recent call last):
  File "/usr/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
Exception in thread Thread-5 (bootstrap_thread):
    self.run()
  File "/usr/lib/python3.10/threading.py", line 953, in run
Traceback (most recent call last):
  File "/usr/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.10/threading.py", line 953, in run
    self._target(*self._args, **self._kwargs)
  File "/sgl-workspace/sglang_deepep/python/sglang/srt/disaggregation/nixl/conn.py", line 351, in bootstrap_thread
    self._target(*self._args, **self._kwargs)
  File "/sgl-workspace/sglang_deepep/python/sglang/srt/disaggregation/nixl/conn.py", line 351, in bootstrap_thread
    self.run()
    required_dst_info_num = int(waiting_req_bytes[10].decode("ascii"))
    required_dst_info_num = int(waiting_req_bytes[10].decode("ascii"))
IndexError: list index out of range
  File "/usr/lib/python3.10/threading.py", line 953, in run
IndexError: list index out of range
    self._target(*self._args, **self._kwargs)
  File "/sgl-workspace/sglang_deepep/python/sglang/srt/disaggregation/nixl/conn.py", line 351, in bootstrap_thread
    required_dst_info_num = int(waiting_req_bytes[10].decode("ascii"))
IndexError: list index out of range

The nixl PREFILL manager should detect the dummy msg by counting waiting_req_bytes, and update status to WaitingForInput. My test is OK for these commands:

PREFILL/TP4

python -m sglang.launch_server --model-path /data/models/DeepSeek-V2-Lite/ --port 40001 --host 127.0.0.1  --tp-size 4  --trust-remote-code --disaggregation-mode prefill --page-size 32 --base-gpu-id 0 --disaggregation-transfer-backend nixl --disable-radix-cache 

DECODE/TP2

python -m sglang.launch_server --model-path /data/models/DeepSeek-V2-Lite/ --port 40100 --host 127.0.0.1  --tp-size 2--trust-remote-code --disaggregation-mode decode --page-size 32 --base-gpu-id 4 --disaggregation-transfer-backend nixl --disable-radix-cache