fix(test): harden test_linux_uses_dot_config against Windows CI tmp_path

[XeonNAS]

Problem

test_linux_uses_dot_config was failing on the test-windows CI job with:

AssertionError: assert 'AppData' not in
'C:\\Users\\runneradmin\\AppData\\Local\\Temp\\pytest-of-runneradmin\\pytest-0\\
test_linux_uses_dot_config0\\.config\\agileforecasting'

The production code was correct. The path correctly ended in
.config\agileforecasting, but the assertion "AppData" not in str(paths.config_dir)
checked the full absolute path string. On Windows CI, pytest's tmp_path is
rooted inside AppData\Local\Temp, so "AppData" appeared in the prefix even
though the Linux config-path logic was working as intended.

Fix

Replace the three vague substring assertions with a single exact pathlib comparison:

assert paths.config_dir == tmp_path / ".config" / "agileforecasting"

This is:

• Stricter — exact match, not substring presence/absence
• Platform-agnostic — result does not depend on where tmp_path is rooted
• More explicit — the intent (Linux config dir = <home>/.config/agileforecasting) is stated directly

The production code in secure_store._config_base() is unchanged.

Test results

• pytest tests/test_windows_compat.py → 16 passed
• pytest --ignore=tests/test_chart_export.py → 96 passed (mirrors Windows CI job)