feat: add dd-trace for Datadog LLM Observability

[dippindots]

Summary

• Adds dd-trace as a root dependency so it is available in the Docker image
• Enables Datadog LLM Observability auto-instrumentation of Anthropic/OpenAI SDK calls via NODE_OPTIONS=--require dd-trace/init (set in the K8s deployment)
• No code changes required — ddtrace patches the SDKs at startup automatically

Deployment

The following env vars are set in the K8s deployment (values.yaml):

NODE_OPTIONS=--require dd-trace/init
DD_LLMOBS_ENABLED=1
DD_LLMOBS_ML_APP=cbioagent
DD_LLMOBS_AGENTLESS_ENABLED=true
DD_SITE=datadoghq.com
DD_API_KEY=<from librechat-credentials-env secret>

Test plan

• Build image and confirm dd-trace is present in node_modules
• Deploy and verify LLM traces (with token/cost data) appear at https://app.datadoghq.com/llm/traces?query=@ml_app:cbioagent

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