fix: verify notifications are sent with the conversationId set (#2278)

This updates `CodexMessageProcessor` so that each notification it sends
for a `EventMsg` from a `CodexConversation` such that:

- The `params` always has an appropriate `conversationId` field.
- The `method` is now includes the name of the `EventMsg` type rather
than using `codex/event` as the `method` type for all notifications. (We
currently prefix the method name with `codex/event/`, but I think that
should go away once we formalize the notification schema in
`wire_format.rs`.)

As part of this, we update `test_codex_jsonrpc_conversation_flow()` to
verify that the `task_finished` notification has made it through the
system instead of sleeping for 5s and "hoping" the server finished
processing the task. Note we have seen some flakiness in some of our
other, similar integration tests, and I expect adding a similar check
would help in those cases, as well.

codex-rs/mcp-server/tests/common/mcp_process.rs

@@ -374,6 +374,33 @@ impl McpProcess {
         }
     }
 
+    pub async fn read_stream_until_notification_message(
+        &mut self,
+        method: &str,
+    ) -> anyhow::Result<JSONRPCNotification> {
+        loop {
+            let message = self.read_jsonrpc_message().await?;
+            eprint!("message: {message:?}");
+
+            match message {
+                JSONRPCMessage::Notification(notification) => {
+                    if notification.method == method {
+                        return Ok(notification);
+                    }
+                }
+                JSONRPCMessage::Request(_) => {
+                    anyhow::bail!("unexpected JSONRPCMessage::Request: {message:?}");
+                }
+                JSONRPCMessage::Error(_) => {
+                    anyhow::bail!("unexpected JSONRPCMessage::Error: {message:?}");
+                }
+                JSONRPCMessage::Response(_) => {
+                    anyhow::bail!("unexpected JSONRPCMessage::Response: {message:?}");
+                }
+            }
+        }
+    }
+
     pub async fn read_stream_until_configured_response_message(
         &mut self,
     ) -> anyhow::Result<String> {