fix(tui): keep Ctrl-C stashed drafts after /clear

[fcoury-oai]

Why

When a user stashes a draft with Ctrl+C, then runs /clear, the fresh chat session loses the in-memory composer history that held the stashed draft. Pressing Up after /clear can then recall an older submitted prompt instead of the draft the user explicitly saved for later.

What Changed

• Record Ctrl+C-cleared composer text through the existing message history path, so it survives the fresh session created by /clear.
• Keep /clear itself out of local slash-command recall so it does not sit ahead of the stashed draft.
• Add regression coverage for the full flow: submit a prompt, stash a later draft with Ctrl+C, run /clear, then recall the stashed draft before the older prompt.

How to Test

1. Start Codex with just c.
2. Submit a short prompt such as ok and wait for the turn to complete.
3. Type a new draft, press Ctrl+C, then run /clear.
4. Press Up and confirm the stashed draft is restored.
5. Press Up again and confirm the older submitted prompt is still reachable after the stashed draft.

Targeted tests:

• cargo test -p codex-tui slash_clear_after_ctrl_c_keeps_stashed_draft_recallable

Manual verification:

• Reproduced the issue in tmux with RUST_LOG=trace just c -c log_dir=...: before the fix, Up after /clear recalled the older submitted prompt.
• Re-tested the same tmux flow after the fix: Up after /clear restored the Ctrl+C-stashed draft.

[canvrno-oai]

Tested and reviewed.