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/tests/suite/request_user_input.rs

@@ -286,6 +286,19 @@ async fn request_user_input_rejected_in_execute_mode() -> anyhow::Result<()> {
     .await
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn request_user_input_rejected_in_code_mode() -> anyhow::Result<()> {
+    assert_request_user_input_rejected("Code", |model| CollaborationMode {
+        mode: ModeKind::Code,
+        settings: Settings {
+            model,
+            reasoning_effort: None,
+            developer_instructions: None,
+        },
+    })
+    .await
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn request_user_input_rejected_in_custom_mode() -> anyhow::Result<()> {
     assert_request_user_input_rejected("Custom", |model| CollaborationMode {