test: add comprehensive tests for sphinx_ext directive (100% coverage)

Cover all untested code paths: _prepare_svg, _build_svg_styles,
_resolve_target, _build_wrapper_classes, _split_length, _align_spec,
setup, and the full run() method with various option combinations
(caption, events, alt, width, height, scale, align, target, class,
figclass, name) plus error handling for invalid imports and render
failures.

Author: fgmacedo