tui: /undo fails with Session is busy during generation

Summary

Running /undo while a response is generating fails with Session is busy. /undo should cancel the active generation and undo the previous message, as it did before the beta.

Environment

• opencode version: 1.18.4
• OS: Darwin 24.5.0 arm64 (macOS)
• Terminal: tmux (TERM_PROGRAM=tmux, TERM=xterm-256color, COLORTERM=truecolor)
• Shell: /bin/zsh
• Install/channel: Homebrew release; user reports this worked before the beta
• Active plugins: ~/.config/opencode/plugins/opencode-anthropic-auth/index.mjs

Reproduction

1. Start an agent response.
2. While the response is still generating, run /undo.
3. Observe the busy-session error.

Expected Behavior

/undo cancels the active generation, then undoes the previous message.

Actual Behavior

/undo fails consistently across sessions with:

Session is busy: ses_<redacted>

It works only after the agent is no longer in progress.

Additional Context

• Reproduces every time across all tested sessions.
• This behavior reportedly worked before the beta.
• Related closed report: #14014 described revert behavior while busy and noted that session.undo already aborted ongoing work.