chore(deps): pin transformers <5.6 (diffusers single-file CLIP incompat)

transformers 5.6 flattened CLIPTextModel (removed the self.text_model wrapper,
hoisted embeddings/encoder/final_layer_norm to the top level). diffusers' single-file
checkpoint loader (create_diffusers_clip_model_from_ldm) still assumes the nested
layout, so loading SD1.5 .safetensors checkpoints fails on 5.6+ with
'CLIPTextModel object has no attribute text_model' and, once that read is shimmed,
'Cannot copy out of meta tensor' (weights never populate the flattened model).

Pin to >=5.5,<5.6 (last pre-flattening release) which keeps both the single-file
and from_pretrained paths working. The invoke-ai/compel fork accepts any 5.x.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Your Name

pyproject.toml

@@ -49,7 +49,7 @@ dependencies = [
   "torch~=2.7.0",                         # torch and related dependencies are loosely pinned, will respect requirement of `diffusers[torch]`
   "torchsde",                             # diffusers needs this for SDE solvers, but it is not an explicit dep of diffusers
   "torchvision",
-  "transformers>=5.9.0",
+  "transformers>=5.5,<5.6",
 
   # Core application dependencies, pinned for reproducible builds.
   "fastapi-events",