Expose observability fields on app conversations

[juanmichelini]

• this PR was tested by a human, me, juanmichelini.

Curl:

Trace with span name:

Signal catching span name:

Signal config just in case:

Summary

• add observability_metadata, observability_tags, and observability_span_name to AppConversationStartRequest
• forward those fields from POST /api/v1/app-conversations into the SDK StartConversationRequest
• merge caller-provided metadata/tags with app-server-generated observability context for both OpenHands and ACP agents

Context

software-agent-sdk already supports these fields on the lower-level /api/conversations path. After the Agent Canvas migration, the /api/v1/app-conversations backend lives in sandbox-server, so this is the migrated home for exposing the same span-name controls to Cloud/API callers.

Companions:

• feat: add cloud conversation observability fields typescript-client#314 updates CloudClient request typing
• Document app conversation observability fields docs#670 documents the REST fields

Testing

• uv run pytest tests/unit/app_server/test_live_status_app_conversation_service.py -q -k 'observability_fields'
• uv run ruff check openhands/app_server/app_conversation/app_conversation_models.py openhands/app_server/app_conversation/live_status_app_conversation_service.py tests/unit/app_server/test_live_status_app_conversation_service.py