fix(tui): let session.background fall through to the session layer - #51070

Issue for this PR

Closes #49216

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Pressing ctrl+b while a foreground subagent (or shell) runs did nothing, even though the hint advertised the shortcut.

session.background is registered twice: once by the prompt and once by the session route. The prompt copy guards on the autocomplete and input focus with a bare return, which stops the keymap command chain — the KeymapCommand.run contract is "return false to continue" — so the session route's unconditional command never ran and the keypress was silently dropped.

The prompt guards now return false, so the chain falls through to the session route's command and the action runs.

How did you verify your code works?

Added packages/tui/test/cli/tui/session-background.test.tsx: ctrl+b dispatches session.background while a textarea is focused (the default also overlaps input.move.left, but the later session layer wins, so the overlap is not the failure), a guard returning false falls through to the earlier layer, and a disabled duplicate does not block the earlier layer. Ran bun test and bun typecheck in packages/tui.

Screenshots / recordings

N/A — keybind behavior, no visual change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR