[app-server] type client response payloads (#20050)

## Why

`pr17088` adds typed server-originated request/response plumbing, but
successful client responses are still erased into bare JSON-RPC `result`
values before app-server can make any typed decision about them.

This precursor PR keeps successful client responses typed until the
outgoing response seam. It is intentionally limited to
protocol/app-server plumbing so the analytics behavior change can review
separately on top.

## What changed

- Add `ClientResponsePayload` as the pre-serialization client response
body type.
- Route app-server successful response paths through the typed payload
seam while preserving existing handler-local analytics behavior.
- Keep `InterruptConversation` JSON-RPC-only because it has no
`ClientResponse` variant.
- Move the new payload conversion tests into a dedicated protocol test
module.

## Verification

- `cargo check -p codex-app-server`
- `cargo test -p codex-app-server-protocol`

codex-rs/app-server/src/codex_message_processor.rs

@@ -43,6 +43,7 @@ use codex_app_server_protocol::CancelLoginAccountResponse;
 use codex_app_server_protocol::CancelLoginAccountStatus;
 use codex_app_server_protocol::ClientRequest;
 use codex_app_server_protocol::ClientResponse;
+use codex_app_server_protocol::ClientResponsePayload;
 use codex_app_server_protocol::CodexErrorInfo;
 use codex_app_server_protocol::CollaborationModeListParams;
 use codex_app_server_protocol::CollaborationModeListResponse;
@@ -2118,7 +2119,7 @@ impl CodexMessageProcessor {
         let result = self
             .exec_one_off_command_inner(request_id.clone(), params)
             .await
-            .map(|()| None::<serde_json::Value>);
+            .map(|()| None::<ClientResponsePayload>);
         self.send_optional_result(request_id, result).await;
     }
 
@@ -2864,7 +2865,6 @@ impl CodexMessageProcessor {
                             response: response.clone(),
                         },
                     );
-
                 listener_task_context
                     .outgoing
                     .send_response(request_id, response)
@@ -3544,7 +3544,7 @@ impl CodexMessageProcessor {
         let result = self
             .thread_rollback_start(&request_id, params)
             .await
-            .map(|()| None::<serde_json::Value>);
+            .map(|()| None::<ClientResponsePayload>);
         self.send_optional_result(request_id, result).await;
     }
 
@@ -4401,6 +4401,7 @@ impl CodexMessageProcessor {
                     permission_profile,
                     reasoning_effort: session_configured.reasoning_effort,
                 };
+
                 self.analytics_events_client.track_response(
                     request_id.connection_id.0,
                     ClientResponse::ThreadResume {
@@ -4408,7 +4409,6 @@ impl CodexMessageProcessor {
                         response: response.clone(),
                     },
                 );
-
                 let connection_id = request_id.connection_id;
                 let token_usage_thread = include_turns.then(|| response.thread.clone());
                 self.outgoing.send_response(request_id, response).await;
@@ -5027,7 +5027,6 @@ impl CodexMessageProcessor {
                 response: response.clone(),
             },
         );
-
         let connection_id = request_id.connection_id;
         let token_usage_thread = include_turns.then(|| response.thread.clone());
         self.outgoing.send_response(request_id, response).await;
@@ -5811,7 +5810,7 @@ impl CodexMessageProcessor {
         request_id: ConnectionRequestId,
         result: Result<Option<T>, JSONRPCErrorError>,
     ) where
-        T: serde::Serialize,
+        T: Into<ClientResponsePayload>,
     {
         match result {
             Ok(Some(response)) => self.outgoing.send_response(request_id, response).await,