Pin transformers and huggingface-hub in main Studio venv (#4788)

Revert the >= loosening from f9c4b08 back to exact pins.
Using transformers>=4.57.6 allows pip to install 5.x into the main
Studio venv, which breaks huggingface_hub imports
(is_offline_mode removed in newer hub versions).

The main venv must stay on transformers==4.57.6 and
huggingface-hub==0.36.2. The 5.x version lives only in .venv_t5/
and is dynamically switched via sys.path at runtime.

studio/backend/requirements/extras-no-deps.txt

@@ -11,6 +11,6 @@ git+https://github.com/meta-pytorch/OpenEnv.git
 # executorch>=1.0.1               # 41.5 MB - no imports in unsloth/zoo/studio
 torch-c-dlpack-ext
 sentence_transformers==5.2.0
-transformers>=4.57.6
+transformers==4.57.6
 pytorch_tokenizers
 kernels

studio/backend/requirements/single-env/constraints.txt

@@ -1,8 +1,8 @@
 # Single-env pins for unsloth + studio + data-designer
 # Keep compatible with unsloth transformers bounds.
-transformers>=4.57.6
+transformers==4.57.6
 trl==0.23.1
-huggingface-hub>=0.36.2
+huggingface-hub==0.36.2
 
 # Studio stack
 datasets==4.3.0