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/backend-client/src/lib.rs

@@ -6,6 +6,9 @@ pub use client::Client;
 pub use client::RequestError;
 pub use client::ThreadUsage;
 pub use client::ThreadUsageBreakdownGroup;
+pub use client::turn_usage::ApiKeyResponseCost;
+pub use client::turn_usage::ApiKeyTurnCost;
+pub use client::turn_usage::ApiKeyTurnCostStatus;
 pub use types::AccountEntry;
 pub use types::AccountsCheckResponse;
 pub use types::CodeTaskDetailsResponse;