Reject request_user_input outside Plan/Pair (#9955)

## Context

Previous work in https://github.com/openai/codex/pull/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.

AGENTS.md

@@ -11,6 +11,7 @@ In the codex-rs folder where the rust code lives:
 - Always collapse if statements per https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if
 - Always inline format! args when possible per https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
 - Use method references over closures when possible per https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_for_method_calls
+- When possible, make `match` statements exhaustive and avoid wildcard arms.
 - When writing tests, prefer comparing the equality of entire objects over fields one by one.
 - When making a change that adds or changes an API, ensure that the documentation in the `docs/` folder is up to date if applicable.
 - If you change `ConfigToml` or nested config types, run `just write-config-schema` to update `codex-rs/core/config.schema.json`.