Introduce a new function to just send user message [Stack 3/3] (#1686)

- MCP server: add send-user-message tool to send user input to a running
Codex session
- Added an integration tests for the happy and sad paths

Changes:
•	Add tool definition and schema.
•	Expose tool in capabilities.
•	Route and handle tool requests with validation.
•	Tests for success, bad UUID, and missing session.


follow‑ups
• Listen path not implemented yet; the tool is present but marked “don’t
use yet” in code comments.
• Session run flag reset: clear running_session_id_set appropriately
after turn completion/errors.

This is the third PR in a stack.
Stack:
Final: #1686
Intermediate: #1751
First: #1750

codex-rs/mcp-server/src/tool_handlers/mod.rs

@@ -0,0 +1 @@
+pub(crate) mod send_message;

codex-rs/mcp-server/src/tool_handlers/send_message.rs

@@ -0,0 +1,124 @@
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use codex_core::Codex;
+use codex_core::protocol::Op;
+use codex_core::protocol::Submission;
+use mcp_types::RequestId;
+use tokio::sync::Mutex;
+use uuid::Uuid;
+
+use crate::mcp_protocol::ConversationSendMessageArgs;
+use crate::mcp_protocol::ConversationSendMessageResult;
+use crate::mcp_protocol::ToolCallResponseResult;
+use crate::message_processor::MessageProcessor;
+
+pub(crate) async fn handle_send_message(
+    message_processor: &MessageProcessor,
+    id: RequestId,
+    arguments: ConversationSendMessageArgs,
+) {
+    let ConversationSendMessageArgs {
+        conversation_id,
+        content: items,
+        parent_message_id: _,
+        conversation_overrides: _,
+    } = arguments;
+
+    if items.is_empty() {
+        message_processor
+            .send_response_with_optional_error(
+                id,
+                Some(ToolCallResponseResult::ConversationSendMessage(
+                    ConversationSendMessageResult::Error {
+                        message: "No content items provided".to_string(),
+                    },
+                )),
+                Some(true),
+            )
+            .await;
+        return;
+    }
+
+    let session_id = conversation_id.0;
+    let Some(codex) = get_session(session_id, message_processor.session_map()).await else {
+        message_processor
+            .send_response_with_optional_error(
+                id,
+                Some(ToolCallResponseResult::ConversationSendMessage(
+                    ConversationSendMessageResult::Error {
+                        message: "Session does not exist".to_string(),
+                    },
+                )),
+                Some(true),
+            )
+            .await;
+        return;
+    };
+
+    let running = {
+        let running_sessions = message_processor.running_session_ids();
+        let mut running_sessions = running_sessions.lock().await;
+        !running_sessions.insert(session_id)
+    };
+
+    if running {
+        message_processor
+            .send_response_with_optional_error(
+                id,
+                Some(ToolCallResponseResult::ConversationSendMessage(
+                    ConversationSendMessageResult::Error {
+                        message: "Session is already running".to_string(),
+                    },
+                )),
+                Some(true),
+            )
+            .await;
+        return;
+    }
+
+    let request_id_string = match &id {
+        RequestId::String(s) => s.clone(),
+        RequestId::Integer(i) => i.to_string(),
+    };
+
+    let submit_res = codex
+        .submit_with_id(Submission {
+            id: request_id_string,
+            op: Op::UserInput { items },
+        })
+        .await;
+
+    if let Err(e) = submit_res {
+        message_processor
+            .send_response_with_optional_error(
+                id,
+                Some(ToolCallResponseResult::ConversationSendMessage(
+                    ConversationSendMessageResult::Error {
+                        message: format!("Failed to submit user input: {e}"),
+                    },
+                )),
+                Some(true),
+            )
+            .await;
+        return;
+    }
+
+    message_processor
+        .send_response_with_optional_error(
+            id,
+            Some(ToolCallResponseResult::ConversationSendMessage(
+                ConversationSendMessageResult::Ok,
+            )),
+            Some(false),
+        )
+        .await;
+}
+
+pub(crate) async fn get_session(
+    session_id: Uuid,
+    session_map: Arc<Mutex<HashMap<Uuid, Arc<Codex>>>>,
+) -> Option<Arc<Codex>> {
+    let guard = session_map.lock().await;
+    guard.get(&session_id).cloned()
+}