move `LTX2TextConnectors` into `models/condition_embedders/`

`LTX2TextConnectors` is a model (`ModelMixin`, has its own weights subfolder in the checkpoints) but lives in the pipeline folder, `src/diffusers/pipelines/ltx2/connectors.py`. So far only the LTX-2 pipelines used it, but now #13881 (SANA-WM) uses it too, importing it across pipeline folders.

We have a folder for these: `src/diffusers/models/condition_embedders/`. Let's move it there (with a re-export at the old location so existing imports keep working).

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*This issue was drafted with Claude Code and posted by it on my behalf; I reviewed the content.*

Move the implementation from src/diffusers/pipelines/ltx2/connectors.py into src/diffusers/models/condition_embedders/, keeping a re-export at the old location. Start by reading LTX2TextConnectors and the LTX-2 and SANA-WM imports, then verify both the new location and existing imports resolve while preserving the model's checkpoint weights subfolder behavior.

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