Write the Test Suite

@isTest
private class MilestoneTriggerTest {
    @isTest
    static void allowsValidStatusTransitionsInBulk() {
        Project__c proj = new Project__c(Name = 'Test Project', Status__c = 'Active');
        insert proj;

        List<Milestone__c> milestones = new List<Milestone__c>();
        for (Integer i = 0; i < 200; i++) {
            milestones.add(new Milestone__c(Project__c = proj.Id, Status__c = 'Not Started'));
        }
        insert milestones;

        for (Milestone__c m : milestones) m.Status__c = 'In Progress';

        Test.startTest();
        update milestones;
        Test.stopTest();

        for (Milestone__c m : [SELECT Status__c FROM Milestone__c WHERE Project__c = :proj.Id]) {
            Assert.areEqual('In Progress', m.Status__c);
        }
    }
}

1. @isTest — marks the test class.
2. private class MilestoneTriggerTest { — the test class.
3. @isTest — marks the test method.
4. static void allowsValidStatusTransitionsInBulk() { — states what's verified.
5. Project__c proj = new Project__c(Name = 'Test Project', Status__c = 'Active'); — test data.
6. insert proj; — saves it.
7. (blank line)
8. List<Milestone__c> milestones = new List<Milestone__c>(); — a list to build in bulk.
9. for (Integer i = 0; i < 200; i++) { — Module 31's "always test at bulk scale" rule.
10. milestones.add(new Milestone__c(Project__c = proj.Id, Status__c = 'Not Started')); — 200 identical Milestones.
11. } — closes the for loop.
12. insert milestones; — one bulk DML statement.
13. (blank line)
14. for (Milestone__c m : milestones) m.Status__c = 'In Progress'; — a valid transition for every Milestone.
15. (blank line)
16. Test.startTest(); — the governor-limit boundary.
17. update milestones; — one bulk update, firing the trigger once with all 200 records.
18. Test.stopTest(); — closes the boundary.
19. (blank line)
20. for (Milestone__c m : [SELECT Status__c FROM Milestone__c WHERE Project__c = :proj.Id]) { — re-queries every Milestone.
21. Assert.areEqual('In Progress', m.Status__c); — confirms every single one transitioned correctly.
22. } — closes the for loop.
23. } — closes the test method.
24. } — closes the test class.

200 Milestones updated in one bulk operation — Module 31\'s "always test at bulk scale" rule, still the very first check for any trigger-bearing object.

@isTest
static void marksProjectAtRiskWhenMilestoneOverdue() {
    Id fakeProjectId = TestHelper.fakeId(Project__c.SObjectType);
    Milestone__c overdue = new Milestone__c(
        Id = TestHelper.fakeId(Milestone__c.SObjectType),
        Project__c = fakeProjectId,
        Status__c = 'In Progress',
        Due_Date__c = Date.today().addDays(-3)
    );

    FakeMilestoneSelector fakeSelector = new FakeMilestoneSelector(new List<Milestone__c>{overdue});
    ProjectHealthService service = new ProjectHealthService(fakeSelector);

    Map<Id, String> result = service.calculateHealth(new Set<Id>{fakeProjectId});

    Assert.areEqual('At Risk', result.get(fakeProjectId));
}

1. @isTest — marks the test method.
2. static void marksProjectAtRiskWhenMilestoneOverdue() { — states what's verified.
3. Id fakeProjectId = TestHelper.fakeId(Project__c.SObjectType); — a fake, unsaved Id.
4. Milestone__c overdue = new Milestone__c( — an in-memory Milestone, never inserted.
5. Id = TestHelper.fakeId(Milestone__c.SObjectType), — another fake Id.
6. Project__c = fakeProjectId, — linked to the fake project.
7. Status__c = 'In Progress', — still open.
8. Due_Date__c = Date.today().addDays(-3) — three days overdue.
9. ); — closes the constructor.
10. (blank line)
11. FakeMilestoneSelector fakeSelector = new FakeMilestoneSelector(new List<Milestone__c>{overdue}); — a test-only stand-in returning hand-built data.
12. ProjectHealthService service = new ProjectHealthService(fakeSelector); — Lesson 5's constructor, injecting the fake.
13. (blank line)
14. Map<Id, String> result = service.calculateHealth(new Set<Id>{fakeProjectId}); — calls the real service logic against fake data.
15. (blank line)
16. Assert.areEqual('At Risk', result.get(fakeProjectId)); — confirms the overdue Milestone correctly marked the project.
17. } — closes the test method.

This is Lesson 5\'s dependency injection paying off directly in the test suite — ProjectHealthService\'s core business rule is verified without inserting a single real record, running dramatically faster than a DML-based test would.

@isTest
static void recalculatesHealthForAllActiveProjects() {
    // ... insert an Active Project with an overdue Milestone

    Test.startTest();
    Database.executeBatch(new RecalculateProjectHealthBatch());
    Test.stopTest(); // forces the batch to actually run

    // assert the Project's Health__c field was updated correctly
}

1. @isTest — marks the test method.
2. static void recalculatesHealthForAllActiveProjects() { — states what's verified.
3. // ... insert an Active Project with an overdue Milestone — test data setup, summarized.
4. (blank line)
5. Test.startTest(); — the governor-limit boundary.
6. Database.executeBatch(new RecalculateProjectHealthBatch()); — starts the batch job.
7. Test.stopTest(); // forces the batch to actually run — Module 31's and Module 38's boundary, forcing the job to completion.
8. (blank line)
9. // assert the Project's Health__c field was updated correctly — the real assertion, summarized.
10. } — closes the test method.

Test.startTest()/Test.stopTest() forcing the batch to run synchronously within the test — Module 31\'s and Module 38\'s exact async-testing pattern, reused one final time for this project\'s nightly safety net.