feat: support traditional JSON-RPC request/response in MCP server

[bolinfest]

This introduces a new set of request types that our codex mcp supports. Note that these do not conform to MCP tool calls so that instead of having to send something like this:

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "id": 42,
  "params": {
    "name": "newConversation",
    "arguments": {
      "model": "gpt-5",
      "approvalPolicy": "on-request"
    }
  }
}

we can send something like this:

{
  "jsonrpc": "2.0",
  "method": "newConversation",
  "id": 42,
  "params": {
    "model": "gpt-5",
    "approvalPolicy": "on-request"
  }
}

Admittedly, this new format is not a valid MCP tool call, but we are OK with that right now. (That is, not everything we might want to request of codex mcp is something that is appropriate for an autonomous agent to do.)

To start, this introduces four request types:

• newConversation
• sendUserMessage
• addConversationListener
• removeConversationListener

The new mcp-server/tests/codex_message_processor_flow.rs shows how these can be used.

The types are defined on the CodexRequest enum, so we introduce a new CodexMessageProcessor that is responsible for dealing with requests from this enum. The top-level MessageProcessor has been updated so that when process_request() is called, it first checks whether the request conforms to CodexRequest and dispatches it to CodexMessageProcessor if so.

Note that I also decided to use camelCase for the on-the-wire format, as that seems to be the convention for MCP.

For the moment, the new protocol is defined in wire_format.rs within the mcp-server crate, but in a subsequent PR, I will probably move it to its own crate to ensure the protocol has minimal dependencies and that we can codegen a schema from it.

---

Stack created with Sapling. Best reviewed with ReviewStack.

• fix: verify notifications are sent with the conversationId set #2278
• -> feat: support traditional JSON-RPC request/response in MCP server #2264

[gpeal]

Suggested change

	let (sender, mut receiver) = oneshot::channel();
	let (cancel_tx, mut cancel_rx) = oneshot::channel();