Feat: request user input tool (#9472)

### Summary
* Add `requestUserInput` tool that the model can use for gather
feedback/asking question mid turn.


### Tool input schema
```
{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "requestUserInput input",
  "type": "object",
  "additionalProperties": false,
  "required": ["questions"],
  "properties": {
    "questions": {
      "type": "array",
      "description": "Questions to show the user (1-3). Prefer 1 unless multiple independent decisions block progress.",
      "minItems": 1,
      "maxItems": 3,
      "items": {
        "type": "object",
        "additionalProperties": false,
        "required": ["id", "header", "question"],
        "properties": {
          "id": {
            "type": "string",
            "description": "Stable identifier for mapping answers (snake_case)."
          },
          "header": {
            "type": "string",
            "description": "Short header label shown in the UI (12 or fewer chars)."
          },
          "question": {
            "type": "string",
            "description": "Single-sentence prompt shown to the user."
          },
          "options": {
            "type": "array",
            "description": "Optional 2-3 mutually exclusive choices. Put the recommended option first and suffix its label with \"(Recommended)\". Only include \"Other\" option if we want to include a free form option. If the question is free form in nature, do not include any option.",
            "minItems": 2,
            "maxItems": 3,
            "items": {
              "type": "object",
              "additionalProperties": false,
              "required": ["value", "label", "description"],
              "properties": {
                "value": {
                  "type": "string",
                  "description": "Machine-readable value (snake_case)."
                },
                "label": {
                  "type": "string",
                  "description": "User-facing label (1-5 words)."
                },
                "description": {
                  "type": "string",
                  "description": "One short sentence explaining impact/tradeoff if selected."
                }
              }
            }
          }
        }
      }
    }
  }
}
```

### Tool output schema
```
{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "requestUserInput output",
  "type": "object",
  "additionalProperties": false,
  "required": ["answers"],
  "properties": {
    "answers": {
      "type": "object",
      "description": "Map of question id to user answer.",
      "additionalProperties": {
        "type": "object",
        "additionalProperties": false,
        "required": ["selected"],
        "properties": {
          "selected": {
            "type": "array",
            "items": { "type": "string" }
          },
          "other": {
            "type": ["string", "null"]
          }
        }
      }
    }
  }
}
```

codex-rs/app-server/tests/suite/v2/request_user_input.rs

@@ -0,0 +1,127 @@
+use anyhow::Result;
+use app_test_support::McpProcess;
+use app_test_support::create_final_assistant_message_sse_response;
+use app_test_support::create_mock_responses_server_sequence;
+use app_test_support::create_request_user_input_sse_response;
+use app_test_support::to_response;
+use codex_app_server_protocol::JSONRPCResponse;
+use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::ServerRequest;
+use codex_app_server_protocol::ThreadStartParams;
+use codex_app_server_protocol::ThreadStartResponse;
+use codex_app_server_protocol::TurnStartParams;
+use codex_app_server_protocol::TurnStartResponse;
+use codex_app_server_protocol::UserInput as V2UserInput;
+use codex_protocol::openai_models::ReasoningEffort;
+use tokio::time::timeout;
+
+const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+async fn request_user_input_round_trip() -> Result<()> {
+    let codex_home = tempfile::TempDir::new()?;
+    let responses = vec![
+        create_request_user_input_sse_response("call1")?,
+        create_final_assistant_message_sse_response("done")?,
+    ];
+    let server = create_mock_responses_server_sequence(responses).await;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let thread_start_id = mcp
+        .send_thread_start_request(ThreadStartParams {
+            model: Some("mock-model".to_string()),
+            ..Default::default()
+        })
+        .await?;
+    let thread_start_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(thread_start_id)),
+    )
+    .await??;
+    let ThreadStartResponse { thread, .. } = to_response(thread_start_resp)?;
+
+    let turn_start_id = mcp
+        .send_turn_start_request(TurnStartParams {
+            thread_id: thread.id.clone(),
+            input: vec![V2UserInput::Text {
+                text: "ask something".to_string(),
+                text_elements: Vec::new(),
+            }],
+            model: Some("mock-model".to_string()),
+            effort: Some(ReasoningEffort::Medium),
+            ..Default::default()
+        })
+        .await?;
+    let turn_start_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(turn_start_id)),
+    )
+    .await??;
+    let TurnStartResponse { turn, .. } = to_response(turn_start_resp)?;
+
+    let server_req = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_request_message(),
+    )
+    .await??;
+    let ServerRequest::ToolRequestUserInput { request_id, params } = server_req else {
+        panic!("expected ToolRequestUserInput request, got: {server_req:?}");
+    };
+
+    assert_eq!(params.thread_id, thread.id);
+    assert_eq!(params.turn_id, turn.id);
+    assert_eq!(params.item_id, "call1");
+    assert_eq!(params.questions.len(), 1);
+
+    mcp.send_response(
+        request_id,
+        serde_json::json!({
+            "answers": {
+                "confirm_path": { "selected": ["yes"], "other": serde_json::Value::Null }
+            }
+        }),
+    )
+    .await?;
+
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("codex/event/task_complete"),
+    )
+    .await??;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_notification_message("turn/completed"),
+    )
+    .await??;
+
+    Ok(())
+}
+
+fn create_config_toml(codex_home: &std::path::Path, server_uri: &str) -> std::io::Result<()> {
+    let config_toml = codex_home.join("config.toml");
+    std::fs::write(
+        config_toml,
+        format!(
+            r#"
+model = "mock-model"
+approval_policy = "untrusted"
+sandbox_mode = "read-only"
+
+model_provider = "mock_provider"
+
+[features]
+collaboration_modes = true
+
+[model_providers.mock_provider]
+name = "Mock provider for test"
+base_url = "{server_uri}/v1"
+wire_api = "responses"
+request_max_retries = 0
+stream_max_retries = 0
+"#
+        ),
+    )
+}