## Why
Fixes#18718.
After rewinding a thread, `/copy` could still copy the latest assistant
response from before the rewind. The transcript cells were rolled back,
but the copy source was a single `last_agent_markdown` cache that was
not synchronized with backtracking, so the visible conversation and
copied content could diverge.
## What changed
`ChatWidget` now keeps a bounded copy history for the most recent 32
assistant responses, keyed by the visible user-turn count. When local
rollback trims transcript cells, the copy cache is trimmed to the same
surviving user-turn count so `/copy` uses the latest visible assistant
response.
If the user rewinds past the retained copy window, `/copy` now reports:
```text
Cannot copy that response after rewinding. Only the most recent 32 responses are available to /copy.
```
The change also adds coverage for copying the latest surviving response
after rollback and for the over-limit rewind message.
## Verification
- Manually resumed a synthetic 35-turn session, rewound within the
retained window, and verified `/copy` copied the surviving response.
- Manually rewound past the retained window and verified `/copy` showed
the 32-response limit message.
- `cargo test -p codex-tui slash_copy`
- `just fix -p codex-tui`
- `cargo insta pending-snapshots`
Note: `cargo test -p codex-tui` currently fails on unrelated model
catalog and snapshot drift around the default model changing to
`gpt-5.4`; the focused `/copy` tests pass after fixing the new test
setup.
Felipe Coury
cebe57b723