Trace LLMP completions in Langfuse

[mbrotos]

Summary

• add an explicit nested Langfuse Generation around LLMP completions
• record request messages, response content, model, and token usage
• add focused coverage and document the tracing behavior

Validation

• focused LLMP client pytest suite
• mypy on aieng
• Ruff format and check

[mbrotos]

CI note: the only failing check is nbqa-ruff on the pre-existing archived notebook playground/energy_case_study/notebooks/01_energy_oil_case_study.ipynb (PLR0917: six positional arguments). That file is outside this PR; uv-lock, Ruff, and mypy pass.