fix(engine): drop disconnected sequences before the prefill pass

[sergey-scherbina]

What

Reap sequences whose client has disconnected before running the prefill pass,
not after. A prompt prefill is the expensive step; if the client is already gone, the
sequence should be dropped before it consumes a forward.

Why

With max_num_seqs = 1 (serialized scheduling, common on memory-constrained Metal),
a large-prompt prefill from a client that has already disconnected would still run to
completion and block the single slot, stalling every subsequent request. Dropping
disconnected sequences up front frees the queue immediately.

Scope

mistralrs-core/src/engine/mod.rs, +12. Self-contained and general (not tied to any
model or backend). Independent of the other PRs in this series.

---

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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 C Header                 23         4454         3116          790          548
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 CUDA                    119        23575        19136         1696         2743
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 Metal Shading Lan|       37        14287        11284         1136         1867
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 (Total)                              304          226           46           32
─────────────────────────────────────────────────────────────────────────────────
 Markdown                129         9703            0         6648         3055
 |- BASH                  61          600          520           47           33
 |- Dockerfile             2            5            5            0            0
 |- JSON                  18          700          700            0            0
 |- PowerShell             3            5            5            0            0
 |- Python                25          830          722            5          103
 |- Rust                  15          437          382            1           54
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 (Total)                            12417         2445         6704         3268
─────────────────────────────────────────────────────────────────────────────────
 Rust                    625       270388       239956         5864        24568
 |- Markdown             397         9504          452         7882         1170
 (Total)                           279892       240408        13746        25738
─────────────────────────────────────────────────────────────────────────────────
 Svelte                   18         1831         1696           50           85
 |- CSS                    1            4            4            0            0
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 (Total)                             2711         2427           74          210
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