fix(metal): back zero-element KV cache buffers with a shared placeholder

[sergey-scherbina]

What

PagedAttention allocates a per-layer KV cache block buffer sized to the layer's
KV. Hybrid models have layers with no KV (the linear-attention / GatedDeltaNet
layers carry a recurrent state instead), so that size is 0. On Metal,
new_private_buffer(0) produces a zero-length buffer that later indexing treats as
invalid. Back those zero-element buffers with a shared 1-element placeholder
(elem_count.max(1)) so the no-KV layers allocate something valid and are simply
never read as KV.

Why

Without this, any hybrid model (e.g. Qwen3.6 qwen3_5_moe) crashes on Metal as soon
as the cache engine sets up the no-KV layers. The change is a harmless general
hardening for the dense path (a layer with KV is unaffected; max(1) is a no-op
there).

Scope

mistralrs-core/src/paged_attention/cache_engine.rs, +22/-5. Self-contained.

This is a prerequisite for #2201 (Qwen3.6 on Metal). It is split out as its own small
PR for reviewability; suggested merge order: this + the engine-reap fix, then #2201,
then the chunked-prefill PR.

---

Part of splitting the Qwen3.6 work into focused, reviewable PRs:

• fix(metal): back zero-element KV cache buffers with a shared placeholder #2206 - zero-element KV cache buffer (Metal prerequisite for Fix Qwen3.6 (qwen3_5 / qwen3_5_moe) on Metal: RMSNorm, AFQ, lm_head, hybrid KV cache #2201)
• fix(engine): drop disconnected sequences before the prefill pass #2207 - reap disconnected sequences before prefill (independent)
• Fix Qwen3.6 (qwen3_5 / qwen3_5_moe) on Metal: RMSNorm, AFQ, lm_head, hybrid KV cache #2201 - Qwen3.6 (qwen3_5 / qwen3_5_moe) model support
• feat(metal): paged chunked prefill, env-tunable with a safe chunk-size floor #2208 - Metal paged chunked prefill

Suggested merge order: #2206 + #2207 -> #2201 -> #2208.

[github-actions]

Code Metrics Report

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 (Total)                              304          226           46           32
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 Markdown                129         9703            0         6648         3055
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 Rust                    625       270388       239956         5864        24568
 |- Markdown             397         9504          452         7882         1170
 (Total)                           279892       240408        13746        25738
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