[codex] Add marketplace/remove app-server RPC (#17751)

## Summary

Add a new app-server `marketplace/remove` RPC on top of the shared
marketplace-remove implementation.

This change:
- adds `MarketplaceRemoveParams` / `MarketplaceRemoveResponse` to the
app-server protocol
- wires the new request through `codex_message_processor`
- reuses the shared core marketplace-remove flow from the stacked
refactor PR
- updates generated schema files and adds focused app-server coverage

## Validation

- `just write-app-server-schema`
- `just fmt`
- heavy compile/test coverage deferred to GitHub CI per request

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -17,6 +17,7 @@ mod external_agent_config;
 mod fs;
 mod initialize;
 mod marketplace_add;
+mod marketplace_remove;
 mod mcp_resource;
 mod mcp_server_elicitation;
 mod mcp_server_status;