fix: remote conversation OTEL session ID not respected

[timon0305]

Summary

Fixes #1390 - Remote conversation OTEL session ID not respected in Laminar tracing.

Problem:
For remote conversations, the OTEL/Laminar observability span was being started on the client side (remote_conversation.py), but LLM calls happen on the server side (conversation_service.py). Since OTEL context doesn't propagate across HTTP/WebSocket boundaries, traces were not properly associated with the conversation session.

Solution:

• Add start_active_span() in conversation_service.py when starting a conversation on the server
• Add end_active_span() in conversation_service.py when deleting a conversation
• Remove client-side span calls in remote_conversation.py since they created orphan traces

Files changed:

• openhands-agent-server/openhands/agent_server/conversation_service.py
• openhands-sdk/openhands/sdk/conversation/impl/remote_conversation.py

Checklist

• If the PR is changing/adding functionality, are there tests to reflect this?
• If there is an example, have you run the example to make sure that it works?
• If there are instructions on how to run the code, have you followed the instructions and made sure that it works?
• If the feature is significant enough to require documentation, is there a PR open on the OpenHands/docs repository with the same branch name?
• Is the github CI passing?

[all-hands-bot]

This PR has critical concurrency issues with the global span manager that will cause span corruption in production. The span lifecycle is also mismatched between start and delete operations.