Guard image decode against oversized (decompression-bomb) images

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Motivation

Closes #28587.

_load_image (python/sglang/srt/utils/common.py) decodes images via Image.open(BytesIO(...)) with no pixel-count guard. The per-model cap (SGLANG_IMAGE_MAX_PIXELS, used by smart_resize in qwen_vl.py) is only applied after PIL has fully decoded the image. So an oversized image — e.g. a 300-DPI rasterized PDF page, or a 12000×12000 PNG (144 M px) — is fully decoded into memory (≈ 432 MB RGB buffer) and only then downscaled.

Effect on a single Qwen3-VL request with such an image:

• a CPU core pinned at ~100% for a long time (the reported case took ~1m50s),
• RSS grows by ~1 GB per request.

With --max-running-requests 256, concurrent oversized-image requests scale memory linearly and can OOM the server before any GPU inference — a pre-inference DoS vector. vLLM short-circuits the same input via PIL's DecompressionBombWarning.

Note: #27451 truncated the exception message for malformed inputs to 100 chars, which fixed the raw-base64-in-logs / log-flood symptom for the malformed path. It does not help here: a valid oversized image never raises, so the decode cost and memory growth remain. See the issue for the full breakdown.

Modification

• Add _check_image_pixels(width, height) in utils/common.py. Image.open only reads the header, so width/height are known before the expensive decode (.convert() / .load()). The check rejects images whose pixel count exceeds the limit with a clear ValueError, before any pixels are decoded.
• Add SGLANG_IMAGE_MAX_DECODE_PIXELS (environ.py), default 89_478_485 (PIL's default decompression-bomb threshold); set 0 to disable.
• Add a CPU unit test (test/registered/unit/utils/test_load_image_guard.py) covering within-limit, over-limit, and disabled cases.

This intentionally only guards the PIL decode path (the bomb vector). Malformed inputs that fail Image.open outright are already handled by #27451.

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / environment-variable reference as needed.

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CI States

Latest PR Test (Base): ❌ Run #27730377137
Latest PR Test (Extra): ❌ Run #27730376966

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