Emit API-key turn cost telemetry from app-server (#39006)

## What changed

- Track app-server turn lifecycle events for OpenAI API-key sessions when OTLP telemetry export is enabled.
- Query the turn-cost endpoint after completed or aborted turns and emit `codex.turn_cost` with the estimated USD cost, interruption state, model, speed, and reasoning effort.
- Preserve provider organization and project headers, wait for all observed responses to be priced, and retry pending or transiently failed queries with bounded tracking.
- Re-probe availability after authentication changes and shut the worker down with other app-server background tasks.

## Testing

- Cover late API-key login, transient probe failures, telemetry after thread removal, and turns with multiple responses.
- Verify turn-cost requests use API-key authentication and provider scope headers and deserialize priced response data.

GitOrigin-RevId: a303162a54798b554fcb2e9e2b372e1cf9f49f39

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -481,6 +481,7 @@ pub(crate) struct ThreadRequestProcessor {
     pub(super) log_db: Option<LogDbLayer>,
     pub(super) background_tasks: TaskTracker,
     pub(super) skills_watcher: Arc<SkillsWatcher>,
+    pub(super) turn_cost_worker: Option<crate::turn_cost_worker::TurnCostWorkerHandle>,
     pub(super) initial_config_warnings: Arc<Vec<ConfigWarningNotification>>,
 }
 
@@ -513,6 +514,7 @@ impl ThreadRequestProcessor {
         state_db: Option<StateDbHandle>,
         log_db: Option<LogDbLayer>,
         skills_watcher: Arc<SkillsWatcher>,
+        turn_cost_worker: Option<crate::turn_cost_worker::TurnCostWorkerHandle>,
         initial_config_warnings: Vec<ConfigWarningNotification>,
     ) -> Self {
         Self {
@@ -532,6 +534,7 @@ impl ThreadRequestProcessor {
             log_db,
             background_tasks: TaskTracker::new(),
             skills_watcher,
+            turn_cost_worker,
             initial_config_warnings: Arc::new(initial_config_warnings),
         }
     }
@@ -1058,6 +1061,7 @@ impl ThreadRequestProcessor {
             fallback_model_provider: self.config.model_provider_id.clone(),
             codex_home: self.config.codex_home.to_path_buf(),
             skills_watcher: Arc::clone(&self.skills_watcher),
+            turn_cost_worker: self.turn_cost_worker.clone(),
         }
     }
 
@@ -1188,6 +1192,7 @@ impl ThreadRequestProcessor {
             fallback_model_provider: self.config.model_provider_id.clone(),
             codex_home: self.config.codex_home.to_path_buf(),
             skills_watcher: Arc::clone(&self.skills_watcher),
+            turn_cost_worker: self.turn_cost_worker.clone(),
         };
         let request_trace = request_context.request_trace();
         let config_manager = self.config_manager.clone();