Fix Colab training failing with "Please install unsloth_zoo"

The Colab no-venv path in setup.sh (added in #4601, modified in #4618)
only installs studio.txt (fastapi, uvicorn, structlog, etc.) into
system Python. It skips install_python_stack entirely via
_SKIP_PYTHON_DEPS=true, which means base.txt (unsloth + unsloth-zoo),
extras.txt, extras-no-deps.txt (trl, transformers, sentence_transformers),
and overrides.txt (torchao) are never installed.

When the training worker subprocess spawns and tries to import unsloth,
unsloth/__init__.py calls importlib.metadata.version("unsloth_zoo")
which raises PackageNotFoundError, surfaced as:
  "Failed to import ML libraries: Please install unsloth_zoo"

Even if unsloth_zoo were installed, the next failure would be trl
(imported at trainer.py:63) since it lives in extras-no-deps.txt.

Fix: install all four requirement groups in the Colab block, matching
what install_python_stack.py does for the normal venv path:
  - base.txt (unsloth + unsloth-zoo) -- fail-fast on error
  - extras.txt (version-stripped to avoid Colab conflicts)
  - extras-no-deps.txt (trl, transformers, sentence_transformers)
  - overrides.txt (torchao)

Only the Colab code path is changed. Non-Colab paths are untouched.

studio/setup.sh

@@ -303,6 +303,26 @@ if [ ! -x "$VENV_DIR/bin/python" ]; then
         sed 's/[><=!~;].*//' "$SCRIPT_DIR/backend/requirements/studio.txt" \
             | grep -v '^#' | grep -v '^$' \
             | pip install -q -r /dev/stdin 2>/dev/null || true
+
+        # Install the same ML package groups that install_python_stack.py
+        # uses for the normal (venv) path.  Without these the training,
+        # inference, and export workers fail at import time with:
+        #   "Failed to import ML libraries: Please install unsloth_zoo"
+        # base.txt must succeed -- it has unsloth + unsloth-zoo.
+        echo "   Installing core ML packages for training..."
+        pip install -q -r "$SCRIPT_DIR/backend/requirements/base.txt" || {
+            echo "Failed to install base ML packages (unsloth + unsloth-zoo)"
+            exit 1
+        }
+        sed 's/[><=!~;].*//' "$SCRIPT_DIR/backend/requirements/extras.txt" \
+            | grep -v '^#' | grep -v '^$' \
+            | pip install -q -r /dev/stdin 2>/dev/null || true
+        # trl, transformers, sentence_transformers live in extras-no-deps.txt
+        pip install -q --no-deps \
+            -r "$SCRIPT_DIR/backend/requirements/extras-no-deps.txt" 2>/dev/null || true
+        pip install -q --force-reinstall \
+            -r "$SCRIPT_DIR/backend/requirements/overrides.txt" 2>/dev/null || true
+
         _COLAB_NO_VENV=true
     else
         echo "❌ ERROR: Virtual environment not found at $VENV_DIR"