feat: add sampler and eta options to Generate node

Support 6 sampling algorithms: euler, heun, dpm++2m, dpm++2m_sde,
euler_a, ddim. Passed via options_json to both t2i and i2i paths.

Fields renamed to sampler_name/sampler_eta for clarity. Detailed
tooltips explain each algorithm and when eta is needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>