[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/schema/json/v2/MarketplaceRemoveResponse.json

@@ -0,0 +1,29 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "AbsolutePathBuf": {
+      "description": "A path that is guaranteed to be absolute and normalized (though it is not guaranteed to be canonicalized or exist on the filesystem).\n\nIMPORTANT: When deserializing an `AbsolutePathBuf`, a base path must be set using [AbsolutePathBufGuard::new]. If no base path is set, the deserialization will fail unless the path being deserialized is already absolute.",
+      "type": "string"
+    }
+  },
+  "properties": {
+    "installedRoot": {
+      "anyOf": [
+        {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        },
+        {
+          "type": "null"
+        }
+      ]
+    },
+    "marketplaceName": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "marketplaceName"
+  ],
+  "title": "MarketplaceRemoveResponse",
+  "type": "object"
+}