fix(serve): refuse over-length prompts; report finish_reason=length on every path (#3718)

[noahgift]

Closes #3718

A prompt that fills the model's context window is now refused whole, with an OpenAI-style 400 (code: context_length_exceeded, both counts in the body). Before this, the wgpu handler capped max_tokens at 4096 and nothing else. The prompt prefilled anyway, the KV cache grew past the trained context, and the reply said finish_reason: "stop".

• context_budget_3718.rs: context_token_budget gives min(requested, ctx - prompt) or refuses. finish_reason_for is the single "length vs stop" rule.
• Every serve path now reports "length" when the budget ran out: wgpu (both copies), CUDA non-stream, CUDA→CPU fallback, and SafeTensors chat.
• Tests: 25 pass. Clippy is clean on default and on --features wgpu. cargo check --features cuda passes.

Quorum: docs/audits/quorum-PMAT-3718.json, 3/3 PASS at 6df7689. Round 1 caught that CUDA still said "stop"; round 2 fixed it.

CI fix carried here (cop CI rule 3, 2026-09-27)

x86-main's mutation section failed with "cargo test failed in an unmutated tree": the cargo-mutants baseline (cargo test -p apr-cli --lib) hit its 300 s timeout at 7434/7435 tests. All 17 test_llm_band tests SHA-256 current_exe() (the 472 MB test binary) with sha2 at opt-level 0 (~11 MB/s). 8d86812 sets sha2 to opt-level 3 in dev (byte-identical to the hunk in #4554, so either may land first): 65 test_llm_band tests now pass in 1.39 s. Deterministic, so it is fixed, not rerun.

🤖 Generated with Claude Code

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=4550 head=a6acb67602a069a9ce6028405f4db20daa661c76 verdict=REFUSE class=Q1 arm_rc=1

Shadow mode: this records a verdict and merges nothing. A refusal
to arm is not a block (§13 adds zero rows to §7) — the pull request is
exactly as green as it was.

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=true disarmed=true)

{
 "ticket": "PMAT-3718",
 "head": "8d86812d9c7f45d75b6a7579e5a2806305c4b7dd",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": true,
  "disarmed": true,
  "note": "auto-merge was armed from an earlier round; disarmed before lanes launched"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "FAIL",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "FAIL",
   "findings": 4
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3718",
 "head": "c2e56c93db48e4a2b607221995471b034094e06c",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 5
  },
  {
   "lane": 3,
   "verdict": "NO-VERDICT",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): NOT agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3718",
 "head": "c2e56c93db48e4a2b607221995471b034094e06c",
 "width": 3,
 "executor": "agy",
 "agreed": false,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 2,
   "verdict": "FAIL",
   "findings": 4
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

quorum-review (AD-04): three PASS — agreed (auto_merge: checked=true was_armed=false disarmed=false)

{
 "ticket": "PMAT-3718",
 "head": "42eb448d14e6982756d6eca5a5d1a6cb7e1b4628",
 "width": 3,
 "executor": "agy",
 "agreed": true,
 "auto_merge": {
  "checked": true,
  "was_armed": false,
  "disarmed": false,
  "note": "auto-merge not armed"
 },
 "lanes": [
  {
   "lane": 1,
   "verdict": "PASS",
   "findings": 1
  },
  {
   "lane": 2,
   "verdict": "PASS",
   "findings": 0
  },
  {
   "lane": 3,
   "verdict": "PASS",
   "findings": 0
  }
 ]
}

[noahgift]

Moved into #4606 (PRCAP fold, cop order 14:02Z). Fold = MOVE: head a6acb67602a069a9ce6028405f4db20daa661c76 is an ancestor of the pushed fold head, so no work is lost. The branch is kept. Agent: aprender-59