refactor(tests): behavioral tests and shared factories

[weklund]

Summary

• Replace brittle mock-heavy orchestration tests with behavioral tests — TestRunUp (13 tests) previously used 10 @patch decorators per test, testing mock wiring rather than real behavior. Now uses a FakeServiceLayer test double that mocks only at the OS boundary (subprocess, HTTP, signals), while real YAML loading, config reading, and pure functions execute for real.
• Consolidate ~50 duplicate helper functions into tests/factories.py — _make_entry (9 files), _make_stack_yaml (5 files), _make_test_catalog (6 files), _make_profile (5 files), and others now live in one shared module.
• Add AAA structure comments (# Arrange, # Act, # Assert) to all non-trivial tests across 17 modified files.

Key new files

File	Purpose
tests/factories.py	Shared data factories (make_entry, make_stack_yaml, etc.)
tests/fakes.py	FakeServiceLayer — configurable test double for OS-boundary functions
tests/unit/conftest.py	Unit-specific fixtures (stack_on_disk, fake_services, pids_dir, logs_dir)

Before → After (TestRunUp example)

# BEFORE: 10 @patch decorators, ~35 lines of mock setup per test
@patch("mlx_stack.core.stack_up.check_local_model_exists", return_value=None)
@patch("mlx_stack.core.stack_up.start_service")
@patch("mlx_stack.core.stack_up.wait_for_healthy")
# ... 7 more @patch lines ...
def test_successful_startup(self, mock_which, mock_get_value, ...11 params...):
    mock_load_catalog.return_value = _make_test_catalog()
    mock_get_value.side_effect = lambda key: {...}.get(key, "")
    mock_which.side_effect = lambda x: f"/usr/local/bin/{x}"
    mock_lock.return_value.__enter__ = MagicMock(return_value=None)
    mock_lock.return_value.__exit__ = MagicMock(return_value=False)
    # ... more mock config ...

# AFTER: 0 @patch decorators, ~3 lines of setup
def test_successful_startup(self, stack_on_disk, fake_services):
    # Arrange — defaults: all services start and pass health check
    # Act
    result = run_up()
    # Assert
    assert all(t.status == "healthy" for t in result.tiers)

Stats

• -577 net lines (2,545 added, 2,528 removed across 20 files)
• 1,481 tests pass (same count, zero regressions)
• 673 → ~180 @patch usages in unit tests (73% reduction)

Test plan

• uv run pytest tests/unit/ -x -q --tb=short — all 1,481 tests pass
• uv run ruff check tests/ — all lint checks pass
• Hybrid TDD verification: temporarily misconfigured FakeServiceLayer to confirm tests fail for the right reasons

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