fix(studio): add tiktoken to .venv_t5 for Qwen tokenizers

Qwen-family models (Qwen3.5, etc.) use a tiktoken-based tokenizer.
When Studio routes these models through the transformers 5.x overlay
(.venv_t5), the tokenizer instantiation fails with:

  ValueError: Couldn't instantiate the backend tokenizer...
  You need to have sentencepiece or tiktoken installed

This happens because .venv_t5 was set up with only transformers and
huggingface_hub (both --no-deps). The main venv usually has tiktoken,
but on Windows it sometimes does not.

Fix: install tiktoken into .venv_t5 in both setup.sh and the runtime
fallback (_ensure_venv_t5_exists). Uses --no-deps in the runtime path
since regex/requests are always present in the main venv.

studio/backend/utils/transformers_version.py

@@ -223,7 +223,7 @@ def _ensure_venv_t5_exists() -> bool:
 
     logger.warning(".venv_t5 not found at %s — installing at runtime", _VENV_T5_DIR)
     os.makedirs(_VENV_T5_DIR, exist_ok = True)
-    for pkg in (f"transformers=={TRANSFORMERS_5_VERSION}", "huggingface_hub==1.3.0"):
+    for pkg in (f"transformers=={TRANSFORMERS_5_VERSION}", "huggingface_hub==1.3.0", "tiktoken"):
         cmd = [
             sys.executable,
             "-m",

studio/setup.sh

@@ -254,6 +254,11 @@ else
     mkdir -p "$VENV_T5_DIR"
     run_quiet "pip install transformers 5.x" pip install --target "$VENV_T5_DIR" --no-deps "transformers==5.3.0"
     run_quiet "pip install huggingface_hub for t5" pip install --target "$VENV_T5_DIR" --no-deps "huggingface_hub==1.3.0"
+    # tiktoken is needed by Qwen-family tokenizers (and others). The main venv
+    # may already have it, but on Windows the main venv sometimes doesn't, and
+    # the tokenizer loads while .venv_t5 is prepended to sys.path. Install it
+    # here so it is always available.
+    run_quiet "pip install tiktoken for t5" pip install --target "$VENV_T5_DIR" "tiktoken"
     echo "✅ Transformers 5.x pre-installed to $VENV_T5_DIR/"
 
     # ── 7. WSL: pre-install GGUF build dependencies ──