Emit turn cost telemetry for ChatGPT sessions (#41944)

## What changed

- Query workspace-visible turn estimates for ChatGPT-authenticated sessions through the appropriate Codex or ChatGPT endpoint.
- Track completed response IDs and emit `codex.turn.cost_microusd` only when the estimate is nonnegative, visible, and covers every observed response.
- Keep error response bodies out of turn-cost worker logs.

## Testing

- Cover both endpoint styles, nullable estimates, settlement validation, exact micro-USD conversion, and response-ID tracking.
- Add an app-server integration test that verifies a settled ChatGPT turn reaches the OTLP metrics exporter with its turn, conversation, and auth-mode attributes.

GitOrigin-RevId: 6ae3f8df729fad52984a0c6efafd8c96c7214e55

codex-rs/backend-client/src/lib.rs

@@ -2,6 +2,8 @@ mod client;
 pub(crate) mod types;
 
 pub use client::AddCreditsNudgeCreditType;
+pub use client::ChatgptThreadTurnCosts;
+pub use client::ChatgptTurnCost;
 pub use client::Client;
 pub use client::RequestError;
 pub use client::ThreadUsage;