Fix flaky login provider picker focus assertion (#410)

[vizansh]

Resolves #410.

This PR implements the second approach outlined in the issue to fix the flaky focus assertion on the login provider picker screen:

Separation of Concerns: Removed the flaky assert search.has_focus check from the data filtering test (test_tui_login_api_provider_picker_filters_by_name_and_display_name), as filtering behavior doesn't rely on focus states. Changed pilot.pause() calls to pilot.wait_for_scheduled_animations() to flush asynchronous filter events reliably.

Dedicated Focus Test: Added a dedicated autofocus test (test_tui_login_provider_search_autofocus) that utilizes native event flushing to test focus state behavior independently.

The isolated tests passed completely clean locally across consecutive test runs.

cc @ShreyanshVaibhaw — since this issue primarily popped up on your environment, could you check out this branch and verify if it permanently kills the flake on your machine?

Bash

git fetch origin
git checkout fix-flaky-tui-focus
uv run pytest "tests/test_tui_app.py::test_tui_login_api_provider_picker_filters_by_name_and_display_name"
uv run pytest "tests/test_tui_app.py::test_tui_login_provider_search_autofocus"
#original test :-
uv run pytest "tests/test_tui_app.py::test_tui_login_api_provider_picker_filters_by_name_and_display_name" -q

[alejandro-ao]

Reviewed in an isolated worktree (see prior review). Test-only change; wait_for_scheduled_animations() is a correct, stronger flush than pause() under Textual 8.2.8; no coverage loss. Re-ran the two affected tests 10x plus the full login subset — all green. Full CI parity (pytest 1053 passed, ruff, format, mypy) verified locally after merging latest main into the branch.

[alejandro-ao]

Re-approving after branch sync with latest main (daab3b9). Content unchanged from prior approval; CI green, login subset re-verified locally.

[alejandro-ao]

Re-approving after sync with main (8c45bff). CI green; change content identical to prior approvals.