Make the unsloth_cli studio tests pass in isolation (#7599)

* Make the unsloth_cli studio tests pass in isolation

Six tests in test_studio_run_parallel_flag.py and one in
test_studio_secure_flag.py only passed in a full-directory run. All of them
reach the in-venv branch of run(), which does `from state.tool_policy import
set_tool_policy`. That module lives under studio/backend, so it only imports
once something has put that directory on sys.path, and nothing in either file
does. They were relying on test_start.py, which calls
ensure_studio_backend_path() and leaks the sys.path entry, or on
test_studio_cloudflare_flag.py, which stubs the module.

Add a stub_tool_policy_state fixture in a new conftest and use it in the seven,
so the state comes from the test rather than from whatever ran first.

Every file in unsloth_cli/tests now passes on its own, and the suite is stable
across four pytest-randomly seeds.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

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

@@ -231,8 +231,6 @@ jobs:
         # 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