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.

codex-rs/core/src/tools/handlers/request_user_input.rs

@@ -36,12 +36,14 @@ impl ToolHandler for RequestUserInputHandler {
             }
         };
 
-        let disallowed_mode = match session.collaboration_mode().await.mode {
-            ModeKind::Execute => Some("Execute"),
-            ModeKind::Custom => Some("Custom"),
-            _ => None,
-        };
-        if let Some(mode_name) = disallowed_mode {
+        let mode = session.collaboration_mode().await.mode;
+        if !matches!(mode, ModeKind::Plan | ModeKind::PairProgramming) {
+            let mode_name = match mode {
+                ModeKind::Code => "Code",
+                ModeKind::Execute => "Execute",
+                ModeKind::Custom => "Custom",
+                ModeKind::Plan | ModeKind::PairProgramming => unreachable!(),
+            };
             return Err(FunctionCallError::RespondToModel(format!(
                 "request_user_input is unavailable in {mode_name} mode"
             )));