Add exec-server RPC timing and process startup tracing (#47680)

## Why

Detached work can keep request spans open beyond response enqueueing. Record explicit phase timings to distinguish request handling from the full span lifetime.

## What changed

- Record server dispatch and response enqueue offsets from request receipt, plus a response enqueue event for HTTP responses that bypass the dispatcher.
- Emit client request enqueue and response receipt events, including success or server-error outcomes and reader-to-caller delay. Carry completion timestamps to the awaiting task so receipt events retain its tracing context.
- Add spans for process startup, sandbox preparation, shell snapshot preparation and capture, and process spawning. Record process identifiers only when nonempty and at most 64 bytes long.

GitOrigin-RevId: 237db898b3284fe8a6a36cb0055b3c7991d8e8a4

codex-rs/exec-server/src/process_telemetry.rs

@@ -1,5 +1,5 @@
-//! Emits bounded process lifecycle telemetry using identity captured at launch.
-//! Reconnects and later process operations must not replace that identity.
+//! Emits process lifecycle telemetry and bounds process identifiers recorded by tracing.
+//! Lifecycle events retain the identity captured at launch across reconnects and later operations.
 
 use std::sync::Arc;
 
@@ -76,3 +76,7 @@ impl ProcessTelemetry {
         );
     }
 }
+
+pub(crate) fn trace_process_id(process_id: &str) -> Option<&str> {
+    (!process_id.is_empty() && process_id.len() <= 64).then_some(process_id)
+}