Move `LTX2Guidance` Guider to `guiders/` Folder (#14558)

* Move LTX2Guidance from /modular_pipelines/ltx2/ to guiders/ to allow LTX-2 guiders to be correctly loaded from checkpoint

* Add regression test for LTX-2 guiders declared as from_pretrained components

Covers the round trip this move fixes: a checkpoint that declares `guider` and
`audio_guider` in `modular_model_index.json` must record a resolvable
(library, class_name) pair and reload its non-default guidance scales. The
existing `test_modular_index_consistency` skips components without a
`pretrained_model_name_or_path`, so `from_config` guiders were uncovered.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Migrate LTX2Guidance docs to guider docs after move

* Improve LTX2Guidance docs and add arg docstrings

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>