test(gsplat): deduplicate rasterize-from-world python helpers

Factor common GaussianSplat3d construction, camera setup, render invocation, and gradient assertions into shared helpers in test_rasterize_from_world.py to reduce repetition while preserving coverage and behavior.

Signed-off-by: Francis Williams <francis@fwilliams.info>
Co-authored-by: Cursor <cursoragent@cursor.com>

Author: fwilliams