feat: support traditional JSON-RPC request/response in MCP server (#2264)

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:

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

we can send something like this:


```json
{
  "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.



---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2264).
* #2278
* __->__ #2264

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

@@ -18,6 +18,10 @@ use codex_mcp_server::mcp_protocol::ConversationCreateArgs;
 use codex_mcp_server::mcp_protocol::ConversationId;
 use codex_mcp_server::mcp_protocol::ConversationSendMessageArgs;
 use codex_mcp_server::mcp_protocol::ToolCallRequestParams;
+use codex_mcp_server::wire_format::AddConversationListenerParams;
+use codex_mcp_server::wire_format::NewConversationParams;
+use codex_mcp_server::wire_format::RemoveConversationListenerParams;
+use codex_mcp_server::wire_format::SendUserMessageParams;
 
 use mcp_types::CallToolRequestParams;
 use mcp_types::ClientCapabilities;
@@ -236,6 +240,47 @@ impl McpProcess {
         .await
     }
 
+    // ---------------------------------------------------------------------
+    // Codex JSON-RPC (non-tool) helpers
+    // ---------------------------------------------------------------------
+    /// Send a `newConversation` JSON-RPC request.
+    pub async fn send_new_conversation_request(
+        &mut self,
+        params: NewConversationParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("newConversation", params).await
+    }
+
+    /// Send an `addConversationListener` JSON-RPC request.
+    pub async fn send_add_conversation_listener_request(
+        &mut self,
+        params: AddConversationListenerParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("addConversationListener", params).await
+    }
+
+    /// Send a `sendUserMessage` JSON-RPC request with a single text item.
+    pub async fn send_send_user_message_request(
+        &mut self,
+        params: SendUserMessageParams,
+    ) -> anyhow::Result<i64> {
+        // Wire format expects variants in camelCase; text item uses external tagging.
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("sendUserMessage", params).await
+    }
+
+    /// Send a `removeConversationListener` JSON-RPC request.
+    pub async fn send_remove_conversation_listener_request(
+        &mut self,
+        params: RemoveConversationListenerParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("removeConversationListener", params)
+            .await
+    }
+
     async fn send_request(
         &mut self,
         method: &str,