Add LLMObs integration tests for Java OpenAI SDK

[ygree]

Motivation

Add system tests for LLM Observability integration with the Java OpenAI SDK, covering chat completions, tool calls, responses API, and embedding interactions.

Changes

• Add a new Java test app (openai_app) built with Gradle/Javalin that implements OpenAI proxy endpoints for
  testing
• Add openai-java.Dockerfile to build and run the Java test app with dd-java-agent
• Bump dd-apm-test-agent from v1.38.0 to v1.42.0
• Add support for /evp_proxy/v4 endpoint in llmobs_requests(); add gzip decompression and MessagePack
  decoding for Java tracer payloads

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed and the CI green, you can merge it!

🛟 #apm-shared-testing 🛟

Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
  • Get a review from R&P team

[github-actions]

CODEOWNERS have been resolved as:

tests/integration_frameworks/utils/vcr-cassettes/openai/test_chat_completion_stream_False_openai_chat_completions_post_424f6218.json  @DataDog/system-tests-core
tests/integration_frameworks/utils/vcr-cassettes/openai/test_chat_completion_tool_call_stream_False_openai_chat_completions_post_ec6364ef.json  @DataDog/system-tests-core
tests/integration_frameworks/utils/vcr-cassettes/openai/test_responses_create_stream_False_openai_responses_post_661cf6eb.json  @DataDog/system-tests-core
utils/build/docker/java/openai-java.Dockerfile                          @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/java/openai_app/.gitignore                           @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/java/openai_app/build.gradle                         @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/java/openai_app/gradle/wrapper/gradle-wrapper.jar    @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/java/openai_app/gradle/wrapper/gradle-wrapper.properties  @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/java/openai_app/gradlew                              @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/java/openai_app/settings.gradle                      @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/java/openai_app/src/main/java/SingleFileServer.java  @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
utils/build/docker/java/openai_app/system_tests_library_version.sh      @DataDog/apm-java @DataDog/asm-java @DataDog/system-tests-core
tests/integration_frameworks/llm/openai/test_openai_llmobs.py           @DataDog/ml-observability
tests/integration_frameworks/llm/utils.py                               @DataDog/ml-observability
utils/_context/_scenarios/integration_frameworks.py                     @DataDog/system-tests-core
utils/docker_fixtures/_test_agent.py                                    @DataDog/system-tests-core

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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