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/src/protocol/common.rs

@@ -357,6 +357,10 @@ client_request_definitions! {
         params: v2::MarketplaceRemoveParams,
         response: v2::MarketplaceRemoveResponse,
     },
+    MarketplaceUpgrade => "marketplace/upgrade" {
+        params: v2::MarketplaceUpgradeParams,
+        response: v2::MarketplaceUpgradeResponse,
+    },
     PluginList => "plugin/list" {
         params: v2::PluginListParams,
         response: v2::PluginListResponse,