fix(studio): lazy-import transformers in model_config to fix 5.x version switch (#4806)

* fix(studio): lazy-import AutoConfig in model_config.py to fix transformers 5.x version switch

Move `from transformers import AutoConfig` from module level to inside
load_model_config() where it is actually used.

model_config.py is transitively imported at module load time via:
  core/inference/__init__ → llama_cpp → utils.models → model_config

In inference subprocesses (mp.spawn), this chain runs before
_activate_transformers_version() can prepend .venv_t5/ to sys.path.
The eager import caches transformers 4.57.6 in sys.modules, and the
subsequent sys.path change has no effect — Python always checks
sys.modules before sys.path.

Making the import lazy ensures transformers is not loaded until after
version activation, so the subprocess picks up the correct version.

* fix(studio): also lazy-import extract_model_size_b in llama_cpp.py

Belt-and-suspenders: make the import that originally triggered the
chain lazy as well, so future module-level AutoConfig additions in
utils.models cannot reintroduce the problem.

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studio/backend/core/inference/llama_cpp.py

@@ -52,8 +52,14 @@ _REPROMPT_MAX_CHARS = 2000
 _SHARD_FULL_RE = re.compile(r"^(.*)-(\d{5})-of-(\d{5})\.gguf$")
 _SHARD_RE = re.compile(r"^(.*)-\d{5}-of-\d{5}\.gguf$")
 
-# Model size extraction (shared with routes/inference.py)
-from utils.models import extract_model_size_b as _extract_model_size_b
+
+# Model size extraction — lazy import to avoid pulling in transformers
+# at module level.  See PR description for the full explanation.
+def _extract_model_size_b(model_id: str):
+    from utils.models import extract_model_size_b
+
+    return extract_model_size_b(model_id)
+
 
 # ── Pre-compiled patterns for tool XML stripping ─────────────
 _TOOL_CLOSED_PATS = [

studio/backend/utils/models/model_config.py

@@ -5,7 +5,6 @@
 Model and LoRA configuration handling
 """
 
-from transformers import AutoConfig
 from dataclasses import dataclass
 from typing import Optional, Dict, Any
 from utils.paths import (
@@ -422,6 +421,7 @@ def load_model_config(
     """
     Load model config with optional authentication control.
     """
+    from transformers import AutoConfig
 
     if token:
         # Explicit token provided - use it