Studio: screen the detached-window test against the Windows blocklist too (#7936)

* Studio: screen the Windows shell-out tests against the Windows blocklist

_BLOCKED_COMMANDS is built at import time from sys.platform, so powershell and
pwsh are absent from it on the Linux-only studio-backend-ci runner. The two
Windows shell-out tests were the only ones in the file that did not fake the
platform, and faking sys.platform would not have worked anyway because the set
is already bound by the time a test runs.

test_cmd_shellout_is_screened_through_mangled_switches therefore asserted that
powershell was blocked while the blocklist it consulted could not contain it,
and failed all 7 parametrisations on every backend CI run. Its sibling
test_detached_windows_stay_launchable passed vacuously for the same reason.

Patch the set the screening actually reads. The screening code itself is
correct: with the Windows blocklist in effect all 7 mangled-switch spellings
block, and the detached-window cases stay launchable.

* Tighten the fixture docstring and say the blocklist is faked too

studio/backend/tests/test_windows_bash_shell.py

@@ -272,6 +272,17 @@ def test_notes_say_where_commands_run(monkeypatch):
     assert "opens a window on the user's desktop" in tools._build_terminal_shell_note()
 
 
+@pytest.fixture
+def _windows_blocklist(monkeypatch):
+    # _BLOCKED_COMMANDS resolves the Windows names at import, so patching
+    # sys.platform is too late; fake the resolved set instead.
+    monkeypatch.setattr(
+        tools,
+        "_BLOCKED_COMMANDS",
+        tools._BLOCKED_COMMANDS_COMMON | tools._BLOCKED_COMMANDS_WIN,
+    )
+
+
 @pytest.mark.parametrize(
     "command",
     [
@@ -284,17 +295,10 @@ def test_notes_say_where_commands_run(monkeypatch):
         r'cmd //c start /d C:/tmp "" powershell -Command ls',
     ],
 )
-def test_cmd_shellout_is_screened_through_mangled_switches(monkeypatch, command):
+def test_cmd_shellout_is_screened_through_mangled_switches(command, _windows_blocklist):
     # Git Bash turns a lone /c into a path, so a model writes //c. That spelling
     # skipped the nested scan, making `cmd //c powershell` reachable where
     # `cmd /c powershell` was blocked, and `start` launches its argument too.
-    # _BLOCKED_COMMANDS resolves the Windows names at import, so patching
-    # sys.platform is too late; fake the resolved set instead.
-    monkeypatch.setattr(
-        tools,
-        "_BLOCKED_COMMANDS",
-        tools._BLOCKED_COMMANDS_COMMON | tools._BLOCKED_COMMANDS_WIN,
-    )
     assert tools._find_blocked_commands(command)
 
 
@@ -307,7 +311,9 @@ def test_cmd_shellout_is_screened_through_mangled_switches(monkeypatch, command)
         "start notepad",
     ],
 )
-def test_detached_windows_stay_launchable(command):
+def test_detached_windows_stay_launchable(command, _windows_blocklist):
     # `start` is the only route to a window on the user's desktop, which the
     # terminal description promises, so screening must not blanket-block cmd.
+    # The blocklist is faked here too, or the Linux runner asserts this against
+    # a set with no powershell in it and cannot see a blanket block at all.
     assert not tools._find_blocked_commands(command)