Regression: interrupted agent messages vanish from conversation history

What version of Codex is running?

HEAD

Which model were you using?

gpt-5

What platform is your computer?

_No response_

What steps can reproduce the bug?

1. Cut off the model while it's streaming output tokens
2. Send another message asking what it was talking about or why it was cut off
3. ->> It has no idea

What is the expected behavior?

The model should know it was cut off and have the partial response in context

What do you see instead?

The conversation in the TUI shows the partial messages, but the model sees gaps in the conversation history.

Additional information

Regression introduced in #2758
cc @aibrahim-oai