fix(examples-py): validate LangChain, Langfuse, and LiteLLM OTel 2.x spans

[Nightingalelyy]

Summary

• make the LangChain example set bounded, marker-aware, credential-compatible, and explicitly flushed, with true callback-driven sync/async/event/completion streaming coverage
• replace the copied Langfuse exporter patch with the editable local instrumentation package and assert exactly six current-SDK source spans
• extend LiteLLM from three scripts to seven scripts covering tool calls, expected errors, async unary/streaming calls, propagated attributes, and shutdown

Validation

• LangChain: 29/29 example processes
• Langfuse: 1/1 example process, exactly 6 translated spans
• LiteLLM: 7/7 example processes
• exact marker: otel2-fix-py-group-16-20260817T102700Z
• Respan MCP: 45 trace trees and 92 full records inspected; one root per trace, exact counts, valid parentage, and no duplicates or orphans
• all six intended LLM stream records report stream=true with complete content

Companion instrumentation PR: respanai/respan#382