Run unsloth_cli/tests in Backend CI (#7598)

unsloth_cli/tests had no CI at all. unsloth_cli/** was a paths trigger and a
ruff target, so the Backend CI job already fired on CLI changes but never ran
these 673 tests, which cover the studio launcher, the pre-exposure gate and the
auth secret writers. Four had been failing on main unnoticed.

Two were stale rather than broken code:

- test_studio_default_exposes_parallel_option pinned the plain --parallel
  default to 1, but #7455 deliberately moved _PARALLEL_DEFAULT_PLAIN to 4 so a
  new chat does not queue behind the previous one. Assert against the constant
  so the two cannot drift again.
- test_reexec_forwards_api_only expected --secure --api-only to re-exec. The
  pre-exposure gate now refuses that combination, because api-only serves no
  login page and the bootstrap deadline does not apply, so a seeded password
  could never be changed. Drop the case and assert the refusal instead.

Two only passed when a built frontend dist happened to be present, which it is
not in a fresh clone or on a runner. Both reach a public-launch path where the
missing-dist gate exits first, so they never got to the backend check and the
run_server call they are about. Stub _find_frontend_dist the way their siblings
already do.

Own step rather than folding into the tests/ discovery: pyproject's testpaths is
tests/, and this suite needs no PYTHONPATH or CUDA spoof, importing neither
unsloth nor torch. Its deps are already installed by the job (pydantic and
uvicorn, which brings click, via studio.txt; pyyaml explicitly).

.github/workflows/studio-backend-ci.yml

@@ -223,6 +223,18 @@ jobs:
             tests/studio/test_is_mlx_dispatch_gate.py \
             tests/studio/test_xpu_spoof_pipeline.py
 
+      - name: CLI tests (unsloth_cli)
+        # unsloth_cli/tests had no CI at all: `unsloth_cli/**` was only a paths
+        # trigger and a ruff target, so 673 tests covering the studio launcher,
+        # the pre-exposure gate and the auth secret writers ran nowhere, and
+        # four of them had been failing on main unnoticed.
+        # Own step, not folded into the tests/ discovery above: pyproject's
+        # testpaths is tests/, and this suite needs no PYTHONPATH or CUDA spoof
+        # (it self-bootstraps sys.path and imports neither unsloth nor torch).
+        # Run the whole directory in one invocation; some files in it are
+        # order-dependent and only pass in a full-directory run.
+        run: python -m pytest unsloth_cli/tests -q --tb=short
+
       - name: Shell installer tests
         # Auto-discovered rather than allowlisted. The old hardcoded list had
         # silently fallen seven files behind tests/run_all.sh, including