[codex] Add token usage to turn tracing spans (#19432)

## Why

Slow Codex turns are easier to debug when token usage is visible in the
trace itself, without joining against separate analytics. This adds
token usage to existing turn-handling spans for regular user turns only.

[Example
turn](https://openai.datadoghq.com/apm/trace/9d353efa2cb5de1f4c5b93dc33c3df04?colorBy=service&graphType=flamegraph&shouldShowLegend=true&sort=time&spanID=3555541504891512675&spanViewType=metadata&traceQuery=)
<img width="1447" height="967" alt="Screenshot 2026-04-24 at 3 03 07 PM"
src="https://github.com/user-attachments/assets/ab7bb187-e7fc-41f0-a366-6c44610b2b2c"
/>

## What Changed

Added response-level token fields on completed handle_responses spans:

gen_ai.usage.input_tokens
gen_ai.usage.cache_read.input_tokens
gen_ai.usage.output_tokens
codex.usage.reasoning_output_tokens
codex.usage.total_tokens
Added aggregate token fields on regular turn spans:

codex.turn.token_usage.*
Added an explicit regular-turn opt-in via
SessionTask::records_turn_token_usage_on_span() so this is not coupled
to span-name strings.

## Testing

- `cargo test -p codex-otel`
- `cargo test -p codex-core
turn_and_completed_response_spans_record_token_usage`
- `just fmt`
- `just fix -p codex-core`
- `just fix -p codex-otel`
- Manual local Electron/app-server smoke test: regular user turn emits
the new span fields

Known status: `cargo test -p codex-core` was attempted and failed in
unrelated existing areas: config approvals, request-permissions,
git-info ordering, and subagent metadata persistence.

codex-rs/core/src/state/turn.rs

@@ -79,17 +79,25 @@ pub(crate) struct RunningTask {
     pub(crate) _timer: Option<codex_otel::Timer>,
 }
 
+pub(crate) struct RemovedTask {
+    pub(crate) records_turn_token_usage_on_span: bool,
+    pub(crate) active_turn_is_empty: bool,
+}
+
 impl ActiveTurn {
     pub(crate) fn add_task(&mut self, task: RunningTask) {
         let sub_id = task.turn_context.sub_id.clone();
         self.tasks.insert(sub_id, task);
     }
 
-    pub(crate) fn remove_task(&mut self, sub_id: &str) -> bool {
-        if let Some(task) = self.tasks.swap_remove(sub_id) {
-            task.handle.detach();
-        }
-        self.tasks.is_empty()
+    pub(crate) fn remove_task(&mut self, sub_id: &str) -> Option<RemovedTask> {
+        let task = self.tasks.swap_remove(sub_id)?;
+        let records_turn_token_usage_on_span = task.task.records_turn_token_usage_on_span();
+        task.handle.detach();
+        Some(RemovedTask {
+            records_turn_token_usage_on_span,
+            active_turn_is_empty: self.tasks.is_empty(),
+        })
     }
 
     pub(crate) fn drain_tasks(&mut self) -> Vec<RunningTask> {