Reject request_user_input outside Plan/Pair

[charley-oai]

Context

Previous work in #9560 only rejected
request_user_input in Execute and Custom modes. Since then, additional modes
(e.g., Code) were added, so the guard should be mode-agnostic.

What changed

• Switch the handler to an allowlist: only Plan and PairProgramming are allowed
• Return the same error for any other mode (including Code)
• Add a Code-mode rejection test alongside the existing Execute/Custom tests

Why

This prevents request_user_input from being used in modes where it is not
intended, even as new modes are introduced.