Mcp protocol (#1715)

- Add typed MCP protocol surface in
`codex-rs/mcp-server/src/mcp_protocol.rs` for `requests`, `responses`,
and `notifications`
- Requests: `NewConversation`, `Connect`, `SendUserMessage`,
`GetConversations`
- Message content parts: `Text`, `Image` (`ImageUrl`/`FileId`, optional
`ImageDetail`), File (`Url`/`Id`/`inline Data`)
- Responses: `ToolCallResponseEnvelope` with optional `isError` and
`structuredContent` variants (`NewConversation`, `Connect`,
`SendUserMessageAccepted`, `GetConversations`)
- Notifications: `InitialState`, `ConnectionRevoked`, `CodexEvent`,
`Cancelled`
- Uniform `_meta` on `notifications` via `NotificationMeta`
(`conversationId`, `requestId`)
- Unit tests validate JSON wire shapes for key
`requests`/`responses`/`notifications`

codex-rs/core/src/config_types.rs

@@ -78,7 +78,7 @@ pub enum HistoryPersistence {
 #[derive(Deserialize, Debug, Clone, PartialEq, Default)]
 pub struct Tui {}
 
-#[derive(Deserialize, Debug, Clone, Copy, PartialEq, Default)]
+#[derive(Deserialize, Debug, Clone, Copy, PartialEq, Default, Serialize)]
 #[serde(rename_all = "kebab-case")]
 pub enum SandboxMode {
     #[serde(rename = "read-only")]