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/ClientRequest.json

@@ -1721,6 +1721,17 @@
       ],
       "type": "object"
     },
+    "MarketplaceUpgradeParams": {
+      "properties": {
+        "marketplaceName": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "type": "object"
+    },
     "McpResourceReadParams": {
       "properties": {
         "server": {
@@ -4904,6 +4915,30 @@
       "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": {