fix(neuron): TP-vision Stage 0 — reject image requests on the TP path

The TP inference path has no vision tower, and the TP dispatch in
chat_completion / inference_stream returns before the VisionUnsupported
guard runs — so an image request to a TP-loaded model (e.g. beast's
tp=2 Qwen3.6-27B) was silently dropped and answered from text alone,
the exact issue-#3 confident-hallucination pattern Stage C killed for
single-GPU.

Add the request_has_images → VisionUnsupported guard to both
chat_completion_tp and inference_tp_stream, before prefill / before the
SSE stream opens, so beast returns a clean 400 vision_unsupported. The
guard is unconditional for now (TP has no tower); Stage 3 makes it
conditional on the TP model's has_vision once real TP-vision lands.

Detection is covered by the existing request_has_images unit test; the
guard itself is cuda-gated (validated by CI's CUDA type-check).

Refs TP-vision plan Stage 0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/scheduled_tasks.lock

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+{"sessionId":"a27586bb-2ca7-4e92-8d82-12f41b39f392","pid":3106893,"procStart":"59753850","acquiredAt":1780571089798}