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).

unsloth_cli/tests/test_studio_password_prompt.py

@@ -626,6 +626,12 @@ def test_studio_default_in_venv_broken_backend_exits_before_stripping_bootstrap(
 
     # Pretend we are already inside the studio venv, with a broken backend.
     monkeypatch.setattr(sys, "prefix", str(tmp_path / "unsloth_studio"))
+    # A built dist is not present in a fresh clone. The missing-frontend gate
+    # runs first and has its own test below; stub it so this one reaches the
+    # backend check it is actually about.
+    monkeypatch.setattr(
+        studio_mod, "_find_frontend_dist", lambda: Path("/fake/studio/frontend/dist")
+    )
 
     def _boom():
         raise ImportError("cannot import backend run.py")

unsloth_cli/tests/test_studio_run_parallel_flag.py

@@ -606,8 +606,8 @@ def test_studio_default_exposes_parallel_option():
     assert "--parallel" in decls
     assert "--n-parallel" in decls
     assert (
-        getattr(opt, "default", None) == 1
-    ), "studio_default --parallel must default to 1 (pre-PR); `run` is 4"
+        getattr(opt, "default", None) == studio_mod._PARALLEL_DEFAULT_PLAIN
+    ), "studio_default --parallel must use _PARALLEL_DEFAULT_PLAIN"
     assert getattr(opt, "min", None) == 1
     assert getattr(opt, "max", None) == 64
 
@@ -679,7 +679,6 @@ def test_api_only_option_is_registered():
     "extra,present",
     [
         (["--api-only"], True),
-        (["--secure", "--api-only"], True),  # secure headless path
         ([], False),
     ],
 )
@@ -691,6 +690,21 @@ def test_reexec_forwards_api_only(monkeypatch, extra, present):
     assert ("--api-only" in argv) is present, argv
 
 
+def test_secure_api_only_is_refused_before_any_reexec(monkeypatch, tmp_path):
+    """`--secure --api-only` used to re-exec; the pre-exposure gate now refuses
+    it, because api-only has no login page and the bootstrap deadline does not
+    apply, so the seeded password could never be changed."""
+    studio_mod = _load_run_command()
+    monkeypatch.setattr(studio_mod, "STUDIO_HOME", tmp_path)
+
+    result, captured = _invoke_run(monkeypatch, _BASE + ["--secure", "--api-only"])
+
+    assert captured == [], captured
+    assert result.exit_code != 0
+    combined = (result.output or "") + (getattr(result, "stderr", "") or "")
+    assert "default admin password was never changed" in combined.lower()
+
+
 @pytest.mark.parametrize("extra,expected", [(["--api-only"], True), ([], False)])
 def test_in_venv_path_passes_api_only_to_run_server(monkeypatch, extra, expected):
     """In-venv path must forward --api-only to run_server(api_only=...)."""

unsloth_cli/tests/test_studio_secure_flag.py

@@ -261,6 +261,11 @@ def test_run_in_venv_passes_secure_and_forces_host(monkeypatch, tmp_path):
 
     fake_venv = tmp_path / "unsloth_studio"
     monkeypatch.setattr(sys, "prefix", str(fake_venv))
+    # A built dist is not present in a fresh clone, and without it the public
+    # launch gate exits before run_server is ever reached.
+    monkeypatch.setattr(
+        studio_mod, "_find_frontend_dist", lambda: Path("/fake/studio/frontend/dist")
+    )
 
     from unsloth_cli import _tool_policy as _tp_mod