## Why
The process-start background task can move to a Tokio worker thread, where the
caller's thread-local tracing subscriber is otherwise unavailable. This can
break propagation of the caller's trace context to the exec-server request.
## What changed
- Attach both the current span and current tracing subscriber to spawned
process-start tasks.
- Run exec-server Bazel unit tests serially because their tracing setup uses
process-global state.
## Testing
Run the trace-context regression test on a multi-thread Tokio runtime so it
exercises propagation across the background task.
GitOrigin-RevId: a586eefd6983916d670fc5a90f0decd0468d273b
jif
54c44b9ed4