[codex] Add marketplace/remove app-server RPC (#17751)

## Summary

Add a new app-server `marketplace/remove` RPC on top of the shared
marketplace-remove implementation.

This change:
- adds `MarketplaceRemoveParams` / `MarketplaceRemoveResponse` to the
app-server protocol
- wires the new request through `codex_message_processor`
- reuses the shared core marketplace-remove flow from the stacked
refactor PR
- updates generated schema files and adds focused app-server coverage

## Validation

- `just write-app-server-schema`
- `just fmt`
- heavy compile/test coverage deferred to GitHub CI per request

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

@@ -48,6 +48,7 @@ use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::ListMcpServerStatusParams;
 use codex_app_server_protocol::LoginAccountParams;
 use codex_app_server_protocol::MarketplaceAddParams;
+use codex_app_server_protocol::MarketplaceRemoveParams;
 use codex_app_server_protocol::McpResourceReadParams;
 use codex_app_server_protocol::McpServerToolCallParams;
 use codex_app_server_protocol::MockExperimentalMethodParams;
@@ -555,6 +556,15 @@ impl McpProcess {
         self.send_request("marketplace/add", params).await
     }
 
+    /// Send a `marketplace/remove` JSON-RPC request.
+    pub async fn send_marketplace_remove_request(
+        &mut self,
+        params: MarketplaceRemoveParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("marketplace/remove", params).await
+    }
+
     /// Send a `plugin/install` JSON-RPC request.
     pub async fn send_plugin_install_request(
         &mut self,