feat(junit-jupiter): add @SharedContainers for cross-class container sharing

[kimjonghun0157]

Problem

When multiple test classes extend a common base class, @Testcontainers restarts containers for each class. This happens because TestcontainersExtension stores containers in a class-scoped ExtensionContext.Store, so even static @Container fields are stopped and restarted at every class boundary.

Resolves #1441

Solution

Introduce @SharedContainers and SharedContainersExtension that store static @Container fields in the root ExtensionContext.Store (context.getRoot().getStore(...)). The root store persists for the entire JVM session, so containers are started at most once and cleaned up automatically via CloseableResource / Ryuk at JVM shutdown.

// Before: containers restart for each test class
@Testcontainers
abstract class BaseTest {
    @Container
    static final PostgreSQLContainer<?> DB = new PostgreSQLContainer<>("postgres:16");
}

// After: container starts once, shared across all subclasses
@SharedContainers
abstract class BaseTest {
    @Container
    static final PostgreSQLContainer<?> DB = new PostgreSQLContainer<>("postgres:16");
}

Changes

File	Description
SharedContainers.java	New annotation (disabledWithoutDocker, parallel options)
SharedContainersExtension.java	Extension storing containers in root store; signalAfterTestToContainersFor() helper consistent with TestcontainersExtension
SharedContainersBaseTest.java	Abstract base for cross-class sharing integration tests
SharedContainersTestClassA/B.java	Verify same container ID across two test classes
SharedContainersExtensionTests.java	Unit tests for disabledWithoutDocker / Docker availability
SharedContainersLifecycleAwareTest.java	Verify beforeTest()/afterTest() callbacks on TestLifecycleAware containers
docs/test_framework_integration/junit_5.md	Document new annotation with usage example

Notes

• Instance fields (@Container non-static) still restart per test method, identical to @Testcontainers
• @SharedContainers and @Testcontainers should not be mixed on the same class hierarchy
• Parallel execution support follows the same limitations as @Testcontainers (sequential only)

🤖 Generated with Claude Code