Streamline plugin, apps, and skills handlers (#19490)

## Why

The plugin, app, and skills handlers had a lot of repeated
`send_error`/`return` branches that made the success path hard to scan.
This slice keeps behavior the same while moving fallible steps into
local response-producing helpers, so the request boundary can send one
result.

## What Changed

- Converted plugin list/install/uninstall handlers in
`codex-rs/app-server/src/codex_message_processor/plugins.rs` to return
`Result<*Response, JSONRPCErrorError>` from helper methods and call
`send_result` once.
- Added local error-mapping helpers for plugin install/uninstall and
marketplace failures.
- Applied the same mechanical shape to app list, skills list/config, and
marketplace add/remove/upgrade handlers in
`codex-rs/app-server/src/codex_message_processor.rs`.

## Verification

- `cargo check -p codex-app-server`
- `cargo test -p codex-app-server --test all v2::app_list --
--test-threads=1`
- `cargo test -p codex-app-server --test all v2::plugin_ --
--test-threads=1`
- `cargo test -p codex-app-server --test all v2::skills_list --
--test-threads=1`

Author: pakrym-oai