feat: leverage elicitations in the MCP server (#1623)

This updates the MCP server so that if it receives an
`ExecApprovalRequest` from the `Codex` session, it in turn sends an [MCP
elicitation](https://modelcontextprotocol.io/specification/draft/client/elicitation)
to the client to ask for the approval decision. Upon getting a response,
it forwards the client's decision via `Op::ExecApproval`.

Admittedly, we should be doing the same thing for
`ApplyPatchApprovalRequest`, but this is our first time experimenting
with elicitations, so I'm inclined to defer wiring that code path up
until we feel good about how this one works.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/1623).
* __->__ #1623
* #1622
* #1621
* #1620

codex-rs/mcp-server/src/lib.rs

@@ -72,7 +72,7 @@ pub async fn run_main(codex_linux_sandbox_exe: Option<PathBuf>) -> IoResult<()>
             while let Some(msg) = incoming_rx.recv().await {
                 match msg {
                     JSONRPCMessage::Request(r) => processor.process_request(r).await,
-                    JSONRPCMessage::Response(r) => processor.process_response(r),
+                    JSONRPCMessage::Response(r) => processor.process_response(r).await,
                     JSONRPCMessage::Notification(n) => processor.process_notification(n),
                     JSONRPCMessage::Error(e) => processor.process_error(e),
                 }