Notebooks smoke: run the install cell the way Colab does (#14058)

## Summary

Two install-cell smoke legs fail on main's scheduled `Notebooks CI`
(2026-10-17 06:39, run 39010742438):
- `nb/Nemotron-3-Nano-30B-A3B_A100.ipynb`
- `nb/Qwen3_5_(4B)_Vision.ipynb`

Both fail like this:

```
error: No virtual environment found; run `uv venv` to create an environment, or pass `--system` to install into a non-virtual environment
FileNotFoundError: [Errno 2] No such file or directory: 'unsloth'
```

Their install cells use `!uv pip install`, and the next line is
`!unsloth install-kernels` (unslothai/notebooks #385). On Colab the uv
lines work because Colab sets `UV_SYSTEM_PYTHON=true`. I checked this on
a live `release-colab-external-images_20261014` runtime, along with its
`COLAB_*` keys. The smoke job installs into the runner's system
interpreter as a stand-in for Colab's, but its install step set neither:
- uv refused to run.
- The notebooks also took their non-Colab branch, since they choose the
branch on any environment key containing `COLAB_`.

Setting only `UV_SYSTEM_PYTHON` is not enough, which I tried first while
working on #14056. The non-Colab branch then left `transformers==4.57.6`
(installed `--no-deps`) beside `huggingface_hub` 1.29, which it refuses
to import with, and the gpt-oss leg lost `triton`. Colab's own branch
reinstalls with dependencies, so the job has to take that branch to test
what Colab users run.

## Change

- **`notebooks-ci.yml`:** the `Run install cell` step sets
`UV_SYSTEM_PYTHON: 'true'` and a `COLAB_SMOKE_SHAPED: '1'` marker, with
a comment explaining why.
- The marker is scoped to that one step, which stops before `from
unsloth import`.
- Its name matches none of the specific `COLAB_*` keys Unsloth reads
(`COLAB_ENVIRONMENT`, `COLAB_IFRAME_HEIGHT`, ...).
  - `disk_utils`' generic `COLAB_` scan never runs inside that step.
- **`tests/notebooks/test_smoke_install_contract.py`:**
`test_the_install_cell_runs_the_way_colab_does` requires a `COLAB_*` key
and `UV_SYSTEM_PYTHON` on every step that runs the converted install
cell.

## Testing

- **`Notebooks CI` with `include_smoke=true` on this branch (run
39017258647):** all eight smoke legs pass, including Nemotron-3-Nano and
Qwen3_5 Vision, which fail on main. GRPO and gpt-oss also still pass.
- **The Nemotron leg did real work rather than skipping:**
  - It printed no `No virtual environment found`.
  - uv installed and uninstalled packages.
- `unsloth install-kernels` ran and chose its torch fallback for
causal_conv1d and mamba_ssm on CPU torch.
  - `OK: imports pass under CUDA spoof`.
- **Contract test:** `test_smoke_install_contract.py` gives 39 passed.
With the workflow change reverted, the new test fails.

The `static + transitive resolve` job is still red in that run. That is
the Colab oracle snapshot drift that has failed on every scheduled run
since at least 2026-09-27, and it is unrelated to this change.

.github/workflows/notebooks-ci.yml

@@ -729,6 +729,15 @@ jobs:
         timeout-minutes: 10
         env:
           NOTEBOOK: ${{ matrix.notebook }}
+          # Run the install cell the way Colab does (checked on a live
+          # release-colab-external-images_20261014 runtime). The notebooks pick their
+          # Colab branch on any environment key containing "COLAB_", and Colab sets
+          # UV_SYSTEM_PYTHON so their `!uv pip install` lines target the system
+          # interpreter. Without both, the cell takes the non-Colab branch and uv stops
+          # with "No virtual environment found". The marker's name matches none of the
+          # COLAB_* keys Unsloth itself reads.
+          COLAB_SMOKE_SHAPED: '1'
+          UV_SYSTEM_PYTHON: 'true'
         run: |
           set -euo pipefail
           # Convert THIS notebook only, and take whatever file the converter

tests/notebooks/test_smoke_install_contract.py

@@ -564,3 +564,27 @@ def test_loading_the_stub_helper_leaves_sys_path_alone():
     needle = "sys.path" + ".insert(0, str(REPO / " + chr(34) + "tests" + chr(34) + "))"
     source = Path(__file__).read_text(encoding = "utf-8")
     assert needle not in source, "a sys.path insert of the tests dir is back in this file"
+
+
+def test_the_install_cell_runs_the_way_colab_does():
+    """The install cell sees what Colab gives it: a COLAB_* key and UV_SYSTEM_PYTHON.
+
+    The notebooks take their Colab branch on any environment key containing "COLAB_", and their
+    `!uv pip install` lines need UV_SYSTEM_PYTHON (Colab sets it) to install into the system
+    interpreter. With neither, Nemotron-3-Nano and Qwen3_5 Vision took the non-Colab branch, uv
+    stopped with "No virtual environment found", and `!unsloth install-kernels` failed as a missing
+    command (2026-10-17 cron). With only UV_SYSTEM_PYTHON, the non-Colab branch left
+    transformers 4.57.6 beside huggingface_hub 1.x and gpt-oss lost triton.
+    """
+    steps = [step for step in _job()["steps"] if "_smoke.py" in str(step.get("run", ""))]
+    assert steps, "no step runs the converted install cell any more; update this guard"
+    for step in steps:
+        env = {**(_job().get("env") or {}), **(step.get("env") or {})}
+        name = step.get("name")
+        assert any(
+            "COLAB_" in key for key in env
+        ), f"step {name!r} runs the install cell without a COLAB_* key"
+        assert str(env.get("UV_SYSTEM_PYTHON", "")).lower() in (
+            "1",
+            "true",
+        ), f"step {name!r} runs the install cell without UV_SYSTEM_PYTHON, which Colab sets"