Prevent Esc from dismissing or rewinding /side

[etraut-openai]

Addresses #22599

Why

/side currently lets Esc return to the parent thread. Multiple users reported that this collides with queued-steer UI that also advertises Esc, so a timing-sensitive keypress can dismiss an ephemeral side chat instead of sending the queued prompt.

After removing that dismissal shortcut, the same Esc path could fall through to main-thread backtrack/edit-previous handling, which is not valid for ephemeral side conversations. This keeps /side out of both global Esc behaviors.

What changed

• Remove Esc from the /side return shortcut matcher while keeping the existing Ctrl+C and Ctrl+D behavior.
• Update side-conversation hints and blocked-command copy to advertise Ctrl+C as the return shortcut.
• Rename the reserved Esc keymap label to describe backtracking only.
• Block backtrack/edit-previous handling while a side conversation is active and report Editing previous prompts is unavailable in side conversations. when that path would have fired.
• Keep composer-owned Esc behavior, such as Vim insert-mode escape, routed locally.
• Refresh focused shortcut assertions and TUI snapshots for the updated footer and new side-conversation error message.

Verification

Manually tested /side use cases and Esc, Ctrl+C, Ctrl+D.