[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-protocol/src/protocol/common.rs

@@ -344,6 +344,10 @@ client_request_definitions! {
         params: v2::MarketplaceAddParams,
         response: v2::MarketplaceAddResponse,
     },
+    MarketplaceRemove => "marketplace/remove" {
+        params: v2::MarketplaceRemoveParams,
+        response: v2::MarketplaceRemoveResponse,
+    },
     PluginList => "plugin/list" {
         params: v2::PluginListParams,
         response: v2::PluginListResponse,

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -3481,6 +3481,21 @@ pub struct MarketplaceAddResponse {
     pub already_added: bool,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct MarketplaceRemoveParams {
+    pub marketplace_name: String,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct MarketplaceRemoveResponse {
+    pub marketplace_name: String,
+    pub installed_root: Option<AbsolutePathBuf>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
@@ -8890,6 +8905,40 @@ mod tests {
         );
     }
 
+    #[test]
+    fn marketplace_remove_response_serializes_nullable_installed_root() {
+        let installed_root = if cfg!(windows) {
+            r"C:\marketplaces\debug"
+        } else {
+            "/tmp/marketplaces/debug"
+        };
+        let installed_root = AbsolutePathBuf::try_from(PathBuf::from(installed_root)).unwrap();
+        let installed_root_json = installed_root.as_path().display().to_string();
+        assert_eq!(
+            serde_json::to_value(MarketplaceRemoveResponse {
+                marketplace_name: "debug".to_string(),
+                installed_root: Some(installed_root),
+            })
+            .unwrap(),
+            json!({
+                "marketplaceName": "debug",
+                "installedRoot": installed_root_json,
+            }),
+        );
+
+        assert_eq!(
+            serde_json::to_value(MarketplaceRemoveResponse {
+                marketplace_name: "debug".to_string(),
+                installed_root: None,
+            })
+            .unwrap(),
+            json!({
+                "marketplaceName": "debug",
+                "installedRoot": null,
+            }),
+        );
+    }
+
     #[test]
     fn codex_error_info_serializes_http_status_code_in_camel_case() {
         let value = CodexErrorInfo::ResponseTooManyFailedAttempts {