fix: change EventMsg enum so every variant takes a single struct (#925)

https://github.com/openai/codex/pull/922 did this for the
`SessionConfigured` enum variant, and I think it is generally helpful to
be able to work with the values as each enum variant as their own type,
so this converts the remaining variants and updates all of the
callsites.

Added a simple unit test to verify that the JSON-serialized version of
`Event` does not have any unexpected nesting.

codex-rs/core/src/mcp_tool_call.rs

@@ -7,6 +7,8 @@ use crate::models::FunctionCallOutputPayload;
 use crate::models::ResponseInputItem;
 use crate::protocol::Event;
 use crate::protocol::EventMsg;
+use crate::protocol::McpToolCallBeginEvent;
+use crate::protocol::McpToolCallEndEvent;
 
 /// Handles the specified tool call dispatches the appropriate
 /// `McpToolCallBegin` and `McpToolCallEnd` events to the `Session`.
@@ -39,12 +41,12 @@ pub(crate) async fn handle_mcp_tool_call(
         }
     };
 
-    let tool_call_begin_event = EventMsg::McpToolCallBegin {
+    let tool_call_begin_event = EventMsg::McpToolCallBegin(McpToolCallBeginEvent {
         call_id: call_id.clone(),
         server: server.clone(),
         tool: tool_name.clone(),
         arguments: arguments_value.clone(),
-    };
+    });
     notify_mcp_tool_call_event(sess, sub_id, tool_call_begin_event).await;
 
     // Perform the tool call.
@@ -53,29 +55,29 @@ pub(crate) async fn handle_mcp_tool_call(
         .await
     {
         Ok(result) => (
-            EventMsg::McpToolCallEnd {
+            EventMsg::McpToolCallEnd(McpToolCallEndEvent {
                 call_id,
                 success: !result.is_error.unwrap_or(false),
                 result: Some(result),
-            },
+            }),
             None,
         ),
         Err(e) => (
-            EventMsg::McpToolCallEnd {
+            EventMsg::McpToolCallEnd(McpToolCallEndEvent {
                 call_id,
                 success: false,
                 result: None,
-            },
+            }),
             Some(e),
         ),
     };
 
     notify_mcp_tool_call_event(sess, sub_id, tool_call_end_event.clone()).await;
-    let EventMsg::McpToolCallEnd {
+    let EventMsg::McpToolCallEnd(McpToolCallEndEvent {
         call_id,
         success,
         result,
-    } = tool_call_end_event
+    }) = tool_call_end_event
     else {
         unimplemented!("unexpected event type");
     };