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/Cargo.lock

@@ -822,6 +822,7 @@ dependencies = [
  "serde",
  "serde_json",
  "shlex",
+ "strum_macros 0.27.2",
  "tempfile",
  "tokio",
  "tokio-test",