feat: introduce ClientRequest::SendUserTurn (#2345)

This adds a new request type, `SendUserTurn`, that makes it possible to
submit a `Op::UserTurn` operation (introduced in #2329) to a
conversation. This PR also adds a new integration test that verifies
that changing from `AskForApproval::UnlessTrusted` to
`AskForApproval::Never` mid-conversation ensures that an elicitation is
no longer sent for running `python3 -c print(42)`.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2345).
* __->__ #2345
* #2329
* #2343
* #2340
* #2338

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

@@ -22,6 +22,7 @@ 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 codex_mcp_server::wire_format::SendUserTurnParams;
 
 use mcp_types::CallToolRequestParams;
 use mcp_types::ClientCapabilities;
@@ -281,6 +282,15 @@ impl McpProcess {
             .await
     }
 
+    /// Send a `sendUserTurn` JSON-RPC request.
+    pub async fn send_send_user_turn_request(
+        &mut self,
+        params: SendUserTurnParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("sendUserTurn", params).await
+    }
+
     async fn send_request(
         &mut self,
         method: &str,