Expose response usage metadata in completion events (#41087)

## What changed

- Parse optional `usage_metadata.amount` values from Responses API completion events without converting their string representation.
- Propagate the metadata through SSE and WebSocket streams, regular turns, and remote compaction.
- Add `usageMetadata` to the app-server `rawResponse/completed` notification and its generated schemas.

## Testing

- Cover missing, null, zero, and high-precision amounts, including completion events emitted during compaction.

GitOrigin-RevId: b16d81e8350eab0dc7ff613d0dc9dbed73041713

codex-rs/core/src/compact.rs

@@ -774,6 +774,7 @@ async fn drain_to_completed(
             Ok(ResponseEvent::Completed {
                 response_id,
                 token_usage,
+                usage_metadata,
                 ..
             }) => {
                 sess.send_event(
@@ -781,6 +782,7 @@ async fn drain_to_completed(
                     EventMsg::RawResponseCompleted(RawResponseCompletedEvent {
                         response_id,
                         token_usage: token_usage.clone(),
+                        usage_metadata,
                     }),
                 )
                 .await;