Fibo Edit: multi-reference conditioning and batching (#14566)

* Fibo Edit: multi-reference conditioning, batching, timesteps

* docs: simplify Fibo Edit page, fix EXAMPLE_DOC_STRING to the VLM prompt flow

* docs: point Fibo Edit at the Gemini prompt-to-JSON block and the 1.5 base/turbo ids

briaai/FIBO-edit-prompt-to-JSON is retired in favor of
briaai/FIBO-edit-gemini-prompt-to-JSON, which handles multiple reference
images and masks. Also corrects the checkpoint ids to their canonical
briaai/Fibo-Edit-1.5-base and briaai/Fibo-Edit-1.5-turbo spellings.

* apply review feedback on reference handling and the attention mask

- collapse the mask branch to `if not attention_mask.all():` in both Fibo pipelines
- encode references via `self.image_processor.preprocess(...)`, dropping the manual
  numpy normalization and the `_vae_safe_size` helper
- normalize `image` into a reference list once in `__call__`, pass it to `check_inputs`

* make style

* make style