## Why
Exec-server JSON-RPC calls can cross local and remote transports, but
trace context stopped at the RPC boundary. That made client and server
work difficult to correlate when diagnosing latency or failures.
## What changed
- Propagate the current W3C trace context on outbound JSON-RPC requests.
- Parent inbound request spans from received trace context.
- Record the received JSON-RPC method on server spans and keep each span
open through response enqueue.
- Add only the OTEL dependencies required by the exec-server crate.
## Stack
Review and land this stack in order:
1. #27466 — trace exec-server JSON-RPC requests **(this PR)**
2. #27467 — record bounded connection, request, and process lifecycle
metrics
3. #27470 — observe remote registration and Noise rendezvous lifecycle
## Validation
- `just test -p codex-exec-server --lib` (153 passed)
- `just bazel-lock-check`
- `just fix -p codex-exec-server`
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