bug(otel): span parenting broken after axum migration #110

Summary

After the actix-web → axum migration (cb87687), OTel span parenting breaks in orchestration mode. Two distinct symptoms observed:

1. mcp.tool_call spans orphaned — created as root spans in separate traces instead of nesting under execute_tool
2. agent.turn spans intermittently missing from traces

Both symptoms are absent on a backup of the pre-axum branch (backup/artifact-infra-local, commit 65e14cc) with identical configs, confirmed via side-by-side testing.

Expected hierarchy

agent.stream → orchestration → orchestration.worker → agent.turn → execute_tool → mcp.tool_call

Actual behavior

• mcp.tool_call appears with parentId: null and a different traceId from the orchestration trace
• execute_tool spans exist in the orchestration trace but have no mcp.tool_call children
• Intermittent: some traces correctly nest mcp.tool_call, most don't
• agent.turn occasionally absent from traces

Likely area

Span propagation across tokio::spawn in WrappedTool::call() (crates/aura/src/tool_wrapper.rs:437-441). The Span::current() capture and instrument() propagation worked under actix-web's current_thread runtime but behaves differently with axum's multi-threaded runtime.

Related

• fix(otel): synchronous force_flush blocks tokio runtime during orchestration #98 — synchronous force_flush deadlocking the tokio runtime is a separate symptom of the same migration. The proposed fix in fix(otel): wrap force_flush in spawn_blocking to avoid deadlock #102 (spawn_blocking) resolves the deadlock but the span orphaning persists independently.