fix(visualization): scale top y-edge by scale_y in draw_clusters

[anxkhn]

In draw_clusters the top y-coordinate of debug cluster boxes and their cells
was multiplied by scale_x, while the matching bottom y-coordinate on the next
line correctly used scale_y:

cx0 *= scale_x
cx1 *= scale_x
cy0 *= scale_x   # top edge - wrong axis
cy1 *= scale_y   # bottom edge

scale_x = image.width / page.size.width and
scale_y = image.height / page.size.height differ whenever the rasterized image
aspect ratio does not match the page aspect ratio (for example rendering/OCR at a
DPI that does not preserve aspect). In that case the top edge of every drawn box
is misplaced, distorting the overlay (and for a box whose scaled top passes its
bottom it can invert).

This changes the two top-edge lines to use scale_y, so both y-edges scale by
scale_y. That matches the pattern already used consistently elsewhere in the
repo (table_structure_model.py, table_structure_model_v2.py,
layout_object_detection_model.py) and the earlier fix for the same axis bug in
the table model (#2287).

The change only affects debug visualization overlays gated behind the debug
settings (visualize_cells / visualize_raw_layout / visualize_layout); the
DoclingDocument conversion output is unchanged, and it is a no-op under uniform
scaling (scale_x == scale_y), so no reference data is regenerated.

A focused regression test captures the coordinates handed to
ImageDraw.rectangle under non-uniform scaling and asserts both y-edges equal
bbox.t/b * scale_y.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/utils/visualization.py	50.00%	1 Missing ⚠️

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[dolfim-ibm]

@anxkhn this looks good but we would prefer to drop this test