test: add Docker-backed framework E2E

[ChoMinGi]

What changed

• add explicit Spring AI and LangChain4j consumer tests that exercise automatic model instrumentation
• start a digest-pinned Langfuse v4.0.0-rc.2 stack with disposable local credentials
• export through a generated local TLS endpoint and read observations back through API v2
• verify root/generation parenting, framework and model metadata, I/O, trace fields, and LangChain4j token usage
• add a manual, credential-free GitHub Actions workflow

The stack is isolated by Compose project, publishes only a random loopback TLS port, and removes its containers, network, and volumes on exit. Direct plaintext Docker export remains tracked in #9.

Verified

• ./e2e/langfuse-docker/run.sh
  • Spring AI trace f43a92d703af763a85e0603a607b8821
  • LangChain4j trace e40043bf5f8e8ec8c6bfd5c48b77fdb1
• both ordinary consumer verify runs (the explicit *DockerIT tests stay out of the default suite)
• ./mvnw -B -ntp clean verify
• shellcheck e2e/langfuse-docker/run.sh
• actionlint .github/workflows/docker-e2e.yml
• git diff --check

No Langfuse Cloud credentials are used by this test path.