Cover FlowMatchEulerDiscreteScheduler, HeliosScheduler, and
LTXEulerAncestralRFScheduler contract behavior, DDIM/DDPM
set_timesteps validation guards, state_dict LoRA conversion paths,
and remote_utils encode/decode helpers.

Co-authored-by: Simon Lynch <[email protected]>

…g_utils

Cover high-blast-radius code paths that previously only had indirect
pipeline coverage:
- CogVideoXDDIMScheduler SNR shift and set_timesteps guard
- CogVideoXDPMScheduler multi-step API (old_pred_original_sample)
- DDPMWuerstchenScheduler float timestep schedule and add_noise
- loading_utils path/URL validation and submodule resolution

Use python -m ruff in check_copies so the commit hook works when ruff
is installed as a Python package but not on PATH.

Co-authored-by: Simon Lynch <[email protected]>

Cover temporal windowing helpers and a fast latent-output inference path
for LTXI2VLongMultiPromptPipeline. Extend state_dict_utils PEFT conversion
tests. Skip check_copies when only non-src/diffusers files change so
test-only PRs are not blocked by pre-existing fork copy drift.

Co-authored-by: Simon Lynch <[email protected]>

Co-authored-by: Simon Lynch <[email protected]>

Co-authored-by: Simon Lynch <[email protected]>