fix(ds4): propagate sampler config in layer split

[Graffioh]

Summary

• propagate the request sampler configuration into DeepSeek4LayerSplitAdapter
• preserve request temperature, top-p, top-k, repetition/frequency/presence penalties, and seed instead of silently retaining the adapter defaults
• reseed the adapter RNG when deterministic sampling is explicitly requested
• add unit coverage for sampler propagation and deterministic reseeding

Stack

Stacked on #503. Base is codex/ds4-rocmfpx-server, so this PR contains only the DeepSeek4 layer-split sampler propagation fix. Retarget to the branch where #503 lands once it is merged.

Problem

LayerSplitBackend::run_from_state() calls adapter_->begin_request(req) before prefill and decode. The Qwen35, Gemma4, and Laguna layer-split adapters copy req.sampler into their request-local sampler state, but the DeepSeek4 adapter discarded the request:

void DeepSeek4LayerSplitAdapter::begin_request(const GenerateRequest & req) {
    (void)req;
}

As a result, DeepSeek4 layer-split requests silently retained the default sampler configuration. A request sent with temperature: 0.01 and seed: 42 still reached run_layer_split_ar_decode() with sampler.temp == 0 under GDB.

The adapter now follows the same contract as the other layer-split implementations:

sampler_ = req.sampler;
if (req.do_sample && sampler_.seed != 0) {
    sampler_rng_.seed(sampler_.seed);
}

A non-zero seed explicitly requests reproducible sampling. Seed zero keeps the RNG's existing nondeterministic state and does not disable temperature sampling.

Validation

• branch rebased directly onto the current fix/feat(ds4): ROCmFPX byte-safe decode + fused-decode path + ds4 optimizations #503 head (6c7ee19)
• git diff --check: passed
• diff is limited to the DeepSeek4 layer-split adapter and its unit test
• pre-fix GDB validation: an HTTP request with temperature: 0.01 reached run_layer_split_ar_decode() with sampler.temp == 0
• post-fix Release/RelWithDebInfo CUDA H200 build: passed
• post-fix request with temperature: 0.01 and seed: 42 returned the coherent response The capital of France is Paris.
• unit coverage verifies temperature/top-p/seed propagation and deterministic RNG reseeding

Follow-up

TO INVESTIGATE: Greedy layer-split decode with temperature: 0 has a separate common-runtime issue: run_layer_split_ar_decode() currently skips logits processing instead of selecting their argmax. That cross-model fix is intentionally kept out of this DeepSeek4-specific change.