Prototype agent lifecycle events (semconv #445) across LangGraph, CrewAI and ADK

[meshailabs]

On open-telemetry/semantic-conventions-genai#445, lmolkova asked for real instrumentation over two or three libraries in this repo as evidence for the candidate gen_ai.agent.paused / gen_ai.agent.checkpointed / gen_ai.agent.resumed events, instead of hand-emitted reference scenarios.

Plan, one PR each:

• LangGraph: extend the langchain instrumentation. Draft up as Add LangGraph durable-execution agent lifecycle events #536.
• CrewAI: fill in the existing skeleton package via CrewAI's event bus (flow persistence, human feedback, checkpoints).
• ADK: ADK ships its own OpenTelemetry tracing, so this would be a plugin rather than an instrumentation package. Open to guidance on whether it belongs in this repo at all.

Candidate event names stay pending the semconv proposal, the same approach as #507 and semantic-conventions-genai#483. Flagging before starting the CrewAI and ADK work in case the direction or the split should differ.