[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/codex_app_server_protocol.schemas.json

@@ -675,6 +675,30 @@
           "title": "Marketplace/addRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/v2/RequestId"
+            },
+            "method": {
+              "enum": [
+                "marketplace/remove"
+              ],
+              "title": "Marketplace/removeRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/MarketplaceRemoveParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Marketplace/removeRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -9333,6 +9357,42 @@
         ],
         "type": "object"
       },
+      "MarketplaceRemoveParams": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "marketplaceName": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "marketplaceName"
+        ],
+        "title": "MarketplaceRemoveParams",
+        "type": "object"
+      },
+      "MarketplaceRemoveResponse": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "installedRoot": {
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/AbsolutePathBuf"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "marketplaceName": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "marketplaceName"
+        ],
+        "title": "MarketplaceRemoveResponse",
+        "type": "object"
+      },
       "McpAuthStatus": {
         "enum": [
           "unsupported",