test: move scenario setup into beforeAll to eliminate inter-test dependencies

[Copilot]

Tests in scenarios.test.js were setting summary inside a test() block, creating an implicit dependency: if that test failed or was skipped, all subsequent assertion tests would silently fail with summary being undefined.

Changes

• beforeAll setup: runScenario() and readCoverageSummary() moved into beforeAll() — a beforeAll failure marks all tests in the suite as failed with a clear error immediately
• Purely assertive tests: Individual test() blocks now only contain assertions; the coverage directory existence check is its own standalone test

// Before: summary set inside a test — downstream tests silently broken if this fails
test("run scenario and collect coverage", () => {
  runScenario(scenarioDir);
  summary = readCoverageSummary(scenarioDir);
});

// After: setup isolated from assertions
beforeAll(() => {
  runScenario(scenarioDir);
  summary = readCoverageSummary(scenarioDir);
});

test("coverage directory was created", () => {
  expect(existsSync(join(scenarioDir, "coverage"))).toBe(true);
});

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