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/tests/suite/v2/mod.rs

@@ -19,6 +19,7 @@ mod fs;
 mod initialize;
 mod marketplace_add;
 mod marketplace_remove;
+mod marketplace_upgrade;
 mod mcp_resource;
 mod mcp_server_elicitation;
 mod mcp_server_status;