Isolate the test suite from real application data and the OS keyring

[sgf36]

Problem

The pytest suite had no isolation from the developer's own application data. app/config.py resolves APP_DATA_DIR once, at import, from EASYPOST_DESKTOP_DATA_DIR or else platformdirs.user_data_dir, and nothing set the variable for pytest (no conftest.py, pytest.ini or pyproject setting). Several suites clear tables outright — tests/test_tracking_states.py runs DELETE FROM trackers, others clear shipments and similar — so python -m pytest tests/ on a machine that also runs the app wiped its local tracker and shipment records for both test and production modes. Importing app.core.client also built ClientManager at module level against the real OS keyring.

Fix

tests/conftest.py, which pytest imports before collecting any test module beside it:

• points EASYPOST_DESKTOP_DATA_DIR at a per-session temporary directory, set unconditionally so a developer's exported override is not inherited;
• installs an in-memory keyring backend (the _ScreenshotKeyring pattern from packaging/make_screenshots.py). It stubs the backend, not module attributes, because a name bound by from x import f is not reached by patching x.f;
• imports app.config and refuses to start the session if its data directory resolved anywhere else — a path computed before the conftest ran cannot be moved, so a warning would not protect anything;
• removes the temporary directory at the end of the session.

Proof by observation

tests/test_suite_isolation.py checks what the app itself does, not the variable conftest set:

Check	Positive control (clean interpreter, no conftest)
PRAGMA database_list after init_db() names a file inside the session directory, not the real per-user one	app.config.DATABASE_PATH is the real platformdirs path, outside the session directory
save_settings() writes settings.json inside the session directory	(same control)
keyring.get_keyring() is the session backend, not a keyring.backends.* class; a credential saved through credential_store lands in it and is read back through app.core.client's by-name binding	keyring.get_keyring() is a keyring.backends.* platform backend
A pytest run that imports app.config first exits non-zero with "Refusing to run the tests"	The identical run without the early import passes

Results

• Full suite locally (Windows, Python 3.14): 602 passed, 6 skipped. The latest green main CI run (bc53957) was 595 passed, 6 skipped on both legs; the difference is exactly the 7 new tests, and --collect-only gives 608 with and without the conftest, so no existing test was dropped.
• SHA-256, size and modified time of every file in the real %LOCALAPPDATA%\EasyPostDesktop and %APPDATA%\EasyPostDesktop were recorded before the run and are identical after it.
• No easypost-tests-* directory is left behind.

No application code changes.

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