[codex] Trace exec-server JSON-RPC requests (#27466)

## 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`

codex-rs/exec-server-protocol/src/rpc.rs

@@ -5,6 +5,7 @@
 
 use std::fmt;
 
+use codex_protocol::protocol::W3cTraceContext;
 use serde::Deserialize;
 use serde::Serialize;
 
@@ -45,6 +46,8 @@ pub struct JSONRPCRequest {
     pub method: String,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub params: Option<serde_json::Value>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub trace: Option<W3cTraceContext>,
 }
 
 /// A notification that does not expect a response.