✅ Fix flaky app_dir and launch tutorial tests under xdist

[YuriiMotov]

Description

I noticed that test-redistribute failed on master (see logs), but passed again on later commits.

After investigating it with Claude Code we found a race between xdist workers. Three tests use the real ~/.config/my-super-cli-app/config.json:

• test_cli_config_doesnt_exist expects the file to not exist
• test_cli_config_exists creates it in a fixture
• test_cli in test_launch/test_tutorial002.py creates it via the tutorial code

xdist spreads tests across workers, so these tests run in parallel from time to time and then may fail.

To fix this I added an app_dir fixture in tests/conftest.py. It patches typer.get_app_dir to return tmp_path, so each test gets its own empty dir.
As a bonus, tests no longer create files in the real ~/.config on developers' machines.

To reproduce the flakiness on master:

for i in $(seq 1 15); do pytest tests/test_tutorial/test_app_dir tests/test_tutorial/test_launch -n 4 -q -o addopts="" | grep failed; done

AI Disclaimer

Used Claude Code (Fable 5.1) to investigate and apply the fix. Reviewed manually

Checklist

• This PR links to a GitHub Discussion for the proposed code change.
• I added tests for the change.
• The new or updated tests fail on the main branch and pass on this PR.
• Coverage stays at 100%.
• The documentation explains the change if needed.