Use Testcontainers for integration tests

[lsh1215]

Description

Adds Testcontainers-backed OpenSearch startup for Java client integration tests so developers and the released integration workflow no longer need to start OpenSearch with Docker Compose before running integrationTest.

The Testcontainers path is enabled by default for the integrationTest task. Existing externally managed clusters remain supported by setting tests.rest.cluster or disabling the test container with -Dtests.opensearch.testcontainers.enabled=false.

The Docker/Testcontainers startup path is limited to the integrationTest task. The released integration CI path remains Ubuntu-only, and the macOS/Windows build and unit-test jobs do not get a Docker runtime requirement.

Closes #1916.

Changes

• Add a JVM-singleton OpenSearchTestContainer helper for integration tests.
• Start the container before OpenSearchRestTestCase initializes its REST client.
• Use HTTPS/basic auth and handle the OpenSearch 2.12+ initial admin password requirement.
• Handle ambiguous OpenSearch 2.x/custom image version tags conservatively and cover that compatibility logic with focused tests.
• Disable disk threshold checks only for the disposable Testcontainers-managed node to avoid Docker host disk watermark state applying cluster.blocks.create_index=true during tests.
• Replace released integration workflow Docker Compose setup with Gradle/Testcontainers startup.
• Keep unreleased/source-build integration workflow on its existing external cluster path by disabling Testcontainers there.
• Document the new default, version/image selection, and external-cluster opt-out.

Verification

• git diff --check
• ./gradlew integrationTest --tests "org.opensearch.client.opensearch.integTest.OpenSearchTestContainerTests"
• ./gradlew spotlessJavaCheck
• ./gradlew integrationTest --tests "org.opensearch.client.opensearch.integTest.restclient.PingAndInfoIT" -Dtests.opensearch.version=3.2.0
• GitHub Actions for PR Use Testcontainers for integration tests #2033: all checks passed, including the released integration matrix.

[reta]

Please let's use official https://github.com/opensearch-project/opensearch-testcontainers

[lsh1215]

Sorry, I missed the official module in the first pass. Thanks for pointing it out.

Updated in 728e3fe.

The integration test container setup now uses org.opensearch.testcontainers.OpenSearchContainer, and the local GenericContainer setup code has been removed.

The remaining Testcontainers dependency is compileOnly only, since OpenSearchContainer extends GenericContainer and javac needs that superclass API.

The current CI failures appear unrelated to the test changes; the failed jobs stopped during Gradle buildSrc dependency resolution.