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/v2/MarketplaceUpgradeParams.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "marketplaceName": {
+      "type": [
+        "string",
+        "null"
+      ]
+    }
+  },
+  "title": "MarketplaceUpgradeParams",
+  "type": "object"
+}

codex-rs/app-server-protocol/schema/json/v2/MarketplaceUpgradeResponse.json

@@ -0,0 +1,51 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "AbsolutePathBuf": {
+      "description": "A path that is guaranteed to be absolute and normalized (though it is not guaranteed to be canonicalized or exist on the filesystem).\n\nIMPORTANT: When deserializing an `AbsolutePathBuf`, a base path must be set using [AbsolutePathBufGuard::new]. If no base path is set, the deserialization will fail unless the path being deserialized is already absolute.",
+      "type": "string"
+    },
+    "MarketplaceUpgradeErrorInfo": {
+      "properties": {
+        "marketplaceName": {
+          "type": "string"
+        },
+        "message": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "marketplaceName",
+        "message"
+      ],
+      "type": "object"
+    }
+  },
+  "properties": {
+    "errors": {
+      "items": {
+        "$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"
+}