What is TestWatcher extension interface in JUnit 6 and when do you use it?

import org.junit.jupiter.api.extension.TestWatcher;

public class SlackNotificationWatcher implements TestWatcher {

    @Override
    public void testSuccessful(ExtensionContext ctx) {
        // Optional: log to metrics dashboard
    }

    @Override
    public void testFailed(ExtensionContext ctx, Throwable cause) {
        // Alert the team on test failure
        String testName = ctx.getDisplayName();
        String error    = cause.getMessage();
        slackClient.sendAlert(
            String.format("Test FAILED: %s%nError: %s", testName, error)
        );
    }

    @Override
    public void testAborted(ExtensionContext ctx, Throwable cause) {
        // Log skipped tests (assumption violated)
    }

    @Override
    public void testDisabled(ExtensionContext ctx, Optional<String> reason) {
        // Track @Disabled tests for tech debt reporting
        techDebtTracker.record(ctx.getDisplayName(),
            reason.orElse("No reason given"));
    }
}

// Register:
@ExtendWith(SlackNotificationWatcher.class)
class CriticalPathTest {
    @Test
    void paymentProcessing() { ... }

    @Test
    @Disabled("Awaiting payment gateway fix")
    void refundProcessing() { ... }
}