fix(codex): steer active turns for /ps guidance

[AaronZ345]

Summary

• steer an active Codex app-server turn with turn/steer instead of starting a second turn
• preserve the active turn ID, buffered text, and prompt preamble while applying /ps guidance
• keep the existing turn/start behavior for idle sessions and reject mismatched steering responses
• make four pre-existing environment-sensitive tests hermetic so the full local suite is reliable

Why

/ps calls AgentSession.Send while the current turn is still running. The Codex app-server implementation previously treated every Send as turn/start, reset the active-turn state, and cleared buffered messages. That race could emit an empty result even though Codex later completed a full answer.

Codex exposes turn/steer for same-turn guidance. This change uses it with the active expectedTurnId, matching Codex's native guidance behavior without turning the guidance into a separate user turn.

Testing

• go test ./...
• go test -race ./agent/codex ./core ./platform/cloud-web
• go vet ./...
• go build ./...
• generated the Codex 0.144.1 app-server JSON schema and verified the turn/steer request and response fields

[AaronZ345]

Hi @chenhg5, I’ve rebased this PR onto the latest main and all five CI checks are green. Could you please take another look and merge it when you have a chance? Thanks!

[AaronZ345]

Superseded by #1665.