chore: introduce OutgoingMessageSender (#1622)

Previous to this change, `MessageProcessor` had a
`tokio::sync::mpsc::Sender<JSONRPCMessage>` as an abstraction for server
code to send a message down to the MCP client. Because `Sender` is cheap
to `clone()`, it was straightforward to make it available to tasks
scheduled with `tokio::task::spawn()`.

This worked well when we were only sending notifications or responses
back down to the client, but we want to add support for sending
elicitations in #1623, which means that we need to be able to send
_requests_ to the client, and now we need a bit of centralization to
ensure all request ids are unique.

To that end, this PR introduces `OutgoingMessageSender`, which houses
the existing `Sender<OutgoingMessage>` as well as an `AtomicI64` to mint
out new, unique request ids. It has methods like `send_request()` and
`send_response()` so that callers do not have to deal with
`JSONRPCMessage` directly, as having to set the `jsonrpc` for each
message was a bit tedious (this cleans up `codex_tool_runner.rs` quite a
bit).

We do not have `OutgoingMessageSender` implement `Clone` because it is
important that the `AtomicI64` is shared across all users of
`OutgoingMessageSender`. As such, `Arc<OutgoingMessageSender>` must be
used instead, as it is frequently shared with new tokio tasks.

As part of this change, we update `message_processor.rs` to embrace
`await`, though we must be careful that no individual handler blocks the
main loop and prevents other messages from being handled.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/1622).
* #1623
* __->__ #1622
* #1621
* #1620

codex-rs/mcp-server/src/outgoing_message.rs

@@ -0,0 +1,129 @@
+use std::sync::atomic::AtomicI64;
+use std::sync::atomic::Ordering;
+
+use codex_core::protocol::Event;
+use mcp_types::JSONRPC_VERSION;
+use mcp_types::JSONRPCError;
+use mcp_types::JSONRPCErrorError;
+use mcp_types::JSONRPCMessage;
+use mcp_types::JSONRPCNotification;
+use mcp_types::JSONRPCRequest;
+use mcp_types::JSONRPCResponse;
+use mcp_types::RequestId;
+use mcp_types::Result;
+use serde::Serialize;
+use tokio::sync::mpsc;
+
+pub(crate) struct OutgoingMessageSender {
+    next_request_id: AtomicI64,
+    sender: mpsc::Sender<OutgoingMessage>,
+}
+
+impl OutgoingMessageSender {
+    pub(crate) fn new(sender: mpsc::Sender<OutgoingMessage>) -> Self {
+        Self {
+            next_request_id: AtomicI64::new(0),
+            sender,
+        }
+    }
+
+    #[allow(dead_code)]
+    pub(crate) async fn send_request(&self, method: &str, params: Option<serde_json::Value>) {
+        let outgoing_message = OutgoingMessage::Request(OutgoingRequest {
+            id: RequestId::Integer(self.next_request_id.fetch_add(1, Ordering::Relaxed)),
+            method: method.to_string(),
+            params,
+        });
+        let _ = self.sender.send(outgoing_message).await;
+    }
+
+    pub(crate) async fn send_response(&self, id: RequestId, result: Result) {
+        let outgoing_message = OutgoingMessage::Response(OutgoingResponse { id, result });
+        let _ = self.sender.send(outgoing_message).await;
+    }
+
+    pub(crate) async fn send_event_as_notification(&self, event: &Event) {
+        #[expect(clippy::expect_used)]
+        let params = Some(serde_json::to_value(event).expect("Event must serialize"));
+        let outgoing_message = OutgoingMessage::Notification(OutgoingNotification {
+            method: "codex/event".to_string(),
+            params,
+        });
+        let _ = self.sender.send(outgoing_message).await;
+    }
+
+    pub(crate) async fn send_error(&self, id: RequestId, error: JSONRPCErrorError) {
+        let outgoing_message = OutgoingMessage::Error(OutgoingError { id, error });
+        let _ = self.sender.send(outgoing_message).await;
+    }
+}
+
+/// Outgoing message from the server to the client.
+pub(crate) enum OutgoingMessage {
+    Request(OutgoingRequest),
+    Notification(OutgoingNotification),
+    Response(OutgoingResponse),
+    Error(OutgoingError),
+}
+
+impl From<OutgoingMessage> for JSONRPCMessage {
+    fn from(val: OutgoingMessage) -> Self {
+        use OutgoingMessage::*;
+        match val {
+            Request(OutgoingRequest { id, method, params }) => {
+                JSONRPCMessage::Request(JSONRPCRequest {
+                    jsonrpc: JSONRPC_VERSION.into(),
+                    id,
+                    method,
+                    params,
+                })
+            }
+            Notification(OutgoingNotification { method, params }) => {
+                JSONRPCMessage::Notification(JSONRPCNotification {
+                    jsonrpc: JSONRPC_VERSION.into(),
+                    method,
+                    params,
+                })
+            }
+            Response(OutgoingResponse { id, result }) => {
+                JSONRPCMessage::Response(JSONRPCResponse {
+                    jsonrpc: JSONRPC_VERSION.into(),
+                    id,
+                    result,
+                })
+            }
+            Error(OutgoingError { id, error }) => JSONRPCMessage::Error(JSONRPCError {
+                jsonrpc: JSONRPC_VERSION.into(),
+                id,
+                error,
+            }),
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub(crate) struct OutgoingRequest {
+    pub id: RequestId,
+    pub method: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub params: Option<serde_json::Value>,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub(crate) struct OutgoingNotification {
+    pub method: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub params: Option<serde_json::Value>,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub(crate) struct OutgoingResponse {
+    pub id: RequestId,
+    pub result: Result,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub(crate) struct OutgoingError {
+    pub error: JSONRPCErrorError,
+    pub id: RequestId,
+}