Add reasoning effort to turn tracing spans (#20060)

Why
#19432 added token usage to the turn and response spans. This follow-up
adds the configured reasoning effort so performance traces can be
filtered by model effort.

[example
trace](https://openai.datadoghq.com/apm/trace/1ff708a87159ff4898bdc8bd6091ec18?graphType=waterfall&shouldShowLegend=true&spanID=6596351544047485652&traceQuery=)
<img width="533" height="434" alt="Screenshot 2026-04-28 at 3 52 12 PM"
src="https://github.com/user-attachments/assets/77ef32fc-d7cd-4eec-87b4-26c6798f1af8"
/>


What Changed
- Adds `codex.turn.reasoning_effort` to the turn span.
- Adds `codex.request.reasoning_effort` to `handle_responses`.
- Extends the span test to cover explicit `high` effort with token
usage.

Testing
- `cargo test -p codex-core
turn_and_completed_response_spans_record_token_usage`
- `cargo test -p codex-otel`
- `just fmt`
- `just fix -p codex-core`
- `just fix -p codex-otel`

codex-rs/core/src/tasks/mod.rs

@@ -366,12 +366,14 @@ impl Session {
         let task_cancellation_token = cancellation_token.child_token();
         // Task-owned turn spans keep a core-owned span open for the
         // full task lifecycle after the submission dispatch span ends.
+        let reasoning_effort = turn_context.effective_reasoning_effort_for_tracing();
         let task_span = info_span!(
             "turn",
             otel.name = span_name,
             thread.id = %self.conversation_id,
             turn.id = %turn_context.sub_id,
             model = %turn_context.model_info.slug,
+            codex.turn.reasoning_effort = %reasoning_effort,
             codex.turn.token_usage.input_tokens = field::Empty,
             codex.turn.token_usage.cached_input_tokens = field::Empty,
             codex.turn.token_usage.non_cached_input_tokens = field::Empty,