[Bug] Attention-DP fill-gate fail-fast can skip the model-parallel status gather on a post-fill rank

Description

PyExecutor._sync_gen_only_benchmark_has_insufficient_kv() gates the model-parallel
status gather on rank-local dynamic state:

if (self.benchmark_req_queues_size <= 0 or self.is_warmup
        or not self._benchmark_fill_phase_active):
    return False

_benchmark_fill_phase_active is seeded from the static benchmark-disagg config but is
cleared per rank when the global fill gate opens (_check_benchmark_disagg_gate,
py_executor.py:3995). A rank that observes the flag clear returns before
_allgather_model_parallel_status(), while a peer that is still filling enters the
gather. The two ranks then run different collectives on the same communicator: the fill
gate gathers a pickled object (MPIDist.tp_allgather) while _can_queue() gathers a raw
MPI.INT64_T (tp_allgather_int64). MPI matches them by order and datatype rather than
failing, so the payloads are silently mismatched — the same failure class already
documented from a live run at py_executor.py:4676-4679.

This is the only site in the tree that gates a collective on rank-local state before
entering it. Its sibling _is_benchmark_disagg_fill_complete() states the required
discipline explicitly (py_executor.py:3903-3905):

Keep going to the shared allgather even when this rank is not done so
model-parallel ranks cannot diverge in collective order.

Steps to reproduce

Two real MPI ranks, with the production PyExecutor method bound to a stub whose dist
is a real MPIDist. Rank 0 keeps _benchmark_fill_phase_active = True; rank 1 clears it.

stub._sync_gen_only_benchmark_has_insufficient_kv([], False)

mpirun -n 2 --oversubscribe python native_vote_probe.py

With the flag voted instead of read, both ranks return and agree (mpirun rc=0;
rank 1 spends 1.577 s waiting at the gather, i.e. it actually participates).

Suggested fix direction

Keep only the rank-uniform benchmark_req_queues_size / is_warmup short-circuit,
carry _benchmark_fill_phase_active in the status tuple, and take the fail-fast decision
from the gathered result. Normal serving (TLLM_BENCHMARK_REQ_QUEUES_SIZE unset) still
returns before any collective, so no steady-state synchronisation is added.

Notes

Related to #17095: this is the fill-gate allgather reported there as the first wait
point. Whether it is the whole original deadlock is not established — on this hardware
(8x L20, sm_89) the disaggregated gen-only run stalls before the GEN fill gate is ever
reached, so the original HangDetector -> MPI_Abort signature was not reproduced.

Start in py_executor.py at _sync_gen_only_benchmark_has_insufficient_kv(), _check_benchmark_disagg_gate(), and _allgather_model_parallel_status(). Run native_vote_probe.py with mpirun -n 2 using a real MPIDist, then verify both ranks enter the shared gather and agree on the fail-fast result. The normal serving path should still return before any collective.