arcTo and ellipse accept negative radii #2626

The Canvas 2D radius-taking methods should reject negative radii, but the arcTo() implementation proceeds with a negative radius and ellipse() only checks for zero before passing its radii into Cairo. arcTo() also handles degenerate points before any radius validation, so a negative value can be silently treated as a line in those cases.

The repository already vendors WPT cases for negative arcTo and ellipse radii in test/wpt/path-objects.yaml; those cases expect INDEX_SIZE_ERR. Could these methods validate their radius arguments before modifying the path, consistently with arc() and the API contract?