test(inst/langchain4j): add mock ChatModel CI test and Ollama manual demo

[jbachorik]

What Does This Do

Adds two demo/test classes to the langchain4j-1.0 instrumentation module:

• MockLlmPipelineTest — JUnit test using a two-turn stub ChatModel (first turn returns a tool call request, second returns the final answer). Exercises CONTEXT_BUILD → AWAITING_INFERENCE → TOOL_EXECUTION → AWAITING_INFERENCE in a single CI-safe run. No external services required.

• OllamaLlmPipelineDemo — standalone main class for generating real JFR recordings against a local Ollama server. Run with the dd-java-agent attached and dd.profiling.context.attributes.llm.phase.enabled=true to produce a recording with llm.agent.phase tags for backend ingestion.

Motivation

The mock test provides a deterministic CI validation that all three instrumentation hooks fire without error. The Ollama demo provides realistic inference latency data for the profiling backend.

Part of PROF-14858. Stacked on #11530 (LC4j module). Final PR in this stack.

Additional Notes

Run command for Ollama demo:

java -javaagent:dd-java-agent.jar \
  -Ddd.profiling.enabled=true \
  -Ddd.profiling.context.attributes.llm.phase.enabled=true \
  -cp <classpath> \
  datadog.trace.instrumentation.langchain4j.demo.OllamaLlmPipelineDemo

Prerequisites: ollama serve && ollama pull llama3

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Jira ticket: PROF-14864

[datadog-datadog-prod-us1-2]

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 15 Pipeline jobs failed

DataDog/apm-reliability/dd-trace-java | muzzle-dep-report     

🔧 Fix in code (Fix with Cursor).

Build failed: Cannot access last() element from an empty List in build.gradle:29

DataDog/apm-reliability/dd-trace-java | muzzle: [4/8]     

🔧 Fix in code (Fix with Cursor).

Muzzle validation failed during instrumentation tasks for multiple versions of langchain4j.

DataDog/apm-reliability/dd-trace-java | spotless     

🔧 Fix in code (Fix with Cursor).

Format violations detected in src/main/java/com/datadog/profiling/ddprof/DatadogProfiler.java. Run './gradlew spotlessApply' to fix.

View all 15 failed jobs.

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