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/README.md

@@ -192,6 +192,7 @@ Example with notification opt-out:
 - `skills/list` — list skills for one or more `cwd` values (optional `forceReload`).
 - `marketplace/add` — add a remote plugin marketplace from an HTTP(S) Git URL, SSH Git URL, or GitHub `owner/repo` shorthand, then persist it into the user marketplace config. Returns the installed root path plus whether the marketplace was already present.
 - `marketplace/remove` — remove a configured marketplace by name from the user marketplace config, and delete its installed marketplace root when one exists.
+- `marketplace/upgrade` — upgrade all configured Git plugin marketplaces, or one named marketplace when `marketplaceName` is provided. Returns selected marketplace names, upgraded roots, and per-marketplace errors.
 - `plugin/list` — list discovered plugin marketplaces and plugin state, including effective marketplace install/auth policy metadata, fail-open `marketplaceLoadErrors` entries for marketplace files that could not be parsed or loaded, and best-effort `featuredPluginIds` for the official curated marketplace. `interface.category` uses the marketplace category when present; otherwise it falls back to the plugin manifest category (**under development; do not call from production clients yet**).
 - `plugin/read` — read one plugin by `marketplacePath` plus `pluginName`, returning marketplace info, a list-style `summary`, manifest descriptions/interface metadata, and bundled skills/apps/MCP server names. Returned plugin skills include their current `enabled` state after local config filtering. Plugin app summaries also include `needsAuth` when the server can determine connector accessibility (**under development; do not call from production clients yet**).
 - `skills/changed` — notification emitted when watched local skill files change.