[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/ClientRequest.json

@@ -1287,6 +1287,17 @@
       ],
       "type": "object"
     },
+    "MarketplaceRemoveParams": {
+      "properties": {
+        "marketplaceName": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "marketplaceName"
+      ],
+      "type": "object"
+    },
     "McpResourceReadParams": {
       "properties": {
         "server": {
@@ -4245,6 +4256,30 @@
       "title": "Marketplace/addRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "marketplace/remove"
+          ],
+          "title": "Marketplace/removeRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/MarketplaceRemoveParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Marketplace/removeRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {