test(canvas): cover _is_out_of_image boundary semantics

[wkentaro]

_is_out_of_image was the sole module-level helper in canvas.py with no direct
unit coverage — every sibling (_normalize_bbox_points, _snap_cursor_pos_for_square,
etc.) is already imported and tested in canvas_test.py. It backs is_out_of_pixmap,
which gates vertex/shape clamping throughout the drag/move/reproject paths, so its
inclusive-at-both-edges boundary convention is load-bearing.

This pins that convention: the top-left (0, 0) and far (width, height) corners
count as inside, an interior point is inside, and a point just past any of the four
edges is out.

Test plan

• uv run pytest tests/unit/widgets/canvas_test.py (114 passed)
• make lint (ruff check, ruff format --check, ty check all clean)

[wkentaro]

This was generated by AI during PR processing.

Verdict: recommend-merge

Test-only change adding test_is_out_of_image, which parametrizes the boundary semantics of labelme._widgets.canvas._is_out_of_image (7 cases: interior/corners inside, and each of the four out-of-bounds directions).

• Verified the assertions against the source: the function uses < 0 / > size, so the far width/height edge is inclusive-inside, exactly as the test asserts and its comment documents.
• No rebase needed: branch is up to date with main (0 commits gained since merge-base, no file overlap), so the existing CI green reflects the merged result.
• /code-review (medium): no findings. Import placement is alphabetical, annotations present, matches repo test conventions.
• CI green across all platforms (ubuntu/macos/windows × 3.12-3.14), lint and cla passing.

No CHANGELOG entry (not user-facing). Ready for maintainer to merge.