Add an optional `region: [x1, y1, x2, y2]` parameter to vision_analyze
(pixel coordinates in the ORIGINAL image space). The crop is applied
with Pillow BEFORE the downscale/embed-cap pipeline, so the cropped
region gets the full resolution budget — a zoom for reading small text
or UI details after a full shot.
- New `_crop_image_region` helper: clamps out-of-bounds coordinates to
the image, rejects zero-area/inverted/malformed regions with an error
naming the actual image dimensions so the model can retry sensibly.
- Wired into both the native fast path (`_vision_analyze_native`) and
the legacy aux-LLM path (`vision_analyze_tool`).
- Schema gains one static optional param (byte-stable thereafter); the
description documents the intended flow: full shot first, then zoom.
- No region supplied = behavior unchanged (regression-guarded).
Tests: tests/tools/test_vision_region.py (11 tests — crop applied,
clamping, zero-area rejection with dims, malformed input, pre-downscale
full-budget zoom, schema shape, handler pass-through, no-region
unchanged). Widened one narrow fake_native stub in test_vision_tools.py
to be kwargs-tolerant.
Ported from: QwenLM/qwen-code zoom-image.ts (Apache-2.0)
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