studio: fix tests turning main CI red/flaky (kill-process, install overrides, UI re-login) (#6419)

* studio: set _stats_logger in kill-process test backend

#6377 added a self._stats_logger cleanup step to _kill_process's finally block.
test_kill_process_records_timestamp_on_actual_kill (added in #6400) builds the
backend via __new__, which bypasses __init__ where _stats_logger is set, so once
both landed on main the test raised AttributeError: 'LlamaCppBackend' object has
no attribute '_stats_logger'. Set _stats_logger on the hand-built backend,
mirroring __init__, so the kill path's finally has the attribute it expects.

* test: assert torchao override step on normal Linux, not overrides.txt

#6400 moved the torchao dependency override from a fixed pin in overrides.txt to
a torch-matched spec installed via --force-reinstall (_select_torchao_spec), and
turned overrides.txt into a comment-only pointer. It updated the Windows variant
(test_windows_only_includes_overrides) to check for --reinstall, but left
test_normal_linux_includes_overrides asserting overrides.txt is installed, which
no longer happens. Check for the override step (--reinstall) instead, matching
the Windows test.

* test(ui): tolerate ERR_ABORTED on /login re-login in shutdown step

The Shutdown step re-logs in after a CLI password rotation that revoked the prior
token. The SPA auth guard can client-side-redirect mid-navigation against the
stale token, aborting page.goto("/login") with net::ERR_ABORTED. It is a race
(passes on main most of the time). Resolve on domcontentloaded and tolerate the
abort, relying on the password-field wait that follows to confirm we reached
/login, matching the wait_until used by the other navigations in this file.

tests/python/test_no_torch_filtering.py

@@ -471,11 +471,15 @@ class TestInstallPythonStackSubprocessMock:
     # -- Normal Linux path (NO_TORCH=False, IS_MACOS=False, IS_WINDOWS=False) --
 
     def test_normal_linux_includes_overrides(self):
-        """Normal Linux: overrides.txt IS called."""
+        """Normal Linux: the torchao override step IS called.
+
+        The override step installs a torch-matched torchao spec via
+        --force-reinstall (uv: --reinstall), not overrides.txt directly.
+        """
         cmds = self._capture_install(no_torch = False, is_macos = False, is_windows = False)
-        assert self._cmds_contain_file(
-            cmds, "overrides.txt"
-        ), "overrides.txt should be called on normal Linux"
+        assert any(
+            "--reinstall" in cmd for cmd in cmds
+        ), "torchao override step (--reinstall) should be called on normal Linux"
 
     def test_normal_linux_includes_triton(self):
         """Normal Linux: triton-kernels.txt IS called."""