Add app-server marketplace upgrade RPC (#19074)

## Summary
- add a v2 `marketplace/upgrade` app-server RPC that mirrors the
existing configured Git marketplace upgrade path
- expose typed request/response/error payloads and regenerate
JSON/TypeScript schema fixtures
- add app-server integration coverage for all, named, already
up-to-date, and invalid marketplace upgrade requests

## Tests
- `just write-app-server-schema`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server marketplace_upgrade`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`
- `just fmt`

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -1380,6 +1380,30 @@
           "title": "Marketplace/removeRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/RequestId"
+            },
+            "method": {
+              "enum": [
+                "marketplace/upgrade"
+              ],
+              "title": "Marketplace/upgradeRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/MarketplaceUpgradeParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Marketplace/upgradeRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -7033,6 +7057,64 @@
       "title": "MarketplaceRemoveResponse",
       "type": "object"
     },
+    "MarketplaceUpgradeErrorInfo": {
+      "properties": {
+        "marketplaceName": {
+          "type": "string"
+        },
+        "message": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "marketplaceName",
+        "message"
+      ],
+      "type": "object"
+    },
+    "MarketplaceUpgradeParams": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "marketplaceName": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "title": "MarketplaceUpgradeParams",
+      "type": "object"
+    },
+    "MarketplaceUpgradeResponse": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "errors": {
+          "items": {
+            "$ref": "#/definitions/MarketplaceUpgradeErrorInfo"
+          },
+          "type": "array"
+        },
+        "selectedMarketplaces": {
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
+        },
+        "upgradedRoots": {
+          "items": {
+            "$ref": "#/definitions/AbsolutePathBuf"
+          },
+          "type": "array"
+        }
+      },
+      "required": [
+        "errors",
+        "selectedMarketplaces",
+        "upgradedRoots"
+      ],
+      "title": "MarketplaceUpgradeResponse",
+      "type": "object"
+    },
     "McpAuthStatus": {
       "enum": [
         "unsupported",