[codex] Refactor marketplace add into shared core flow (#17717)

## Summary

Move `codex marketplace add` onto a shared core implementation so the
CLI and app-server path can use one source of truth.

This change:
- adds shared marketplace-add orchestration in `codex-core`
- switches the CLI command to call that shared implementation
- removes duplicated CLI-only marketplace add helpers
- preserves focused parser and add-path coverage while moving the shared
behavior into core tests

## Why

The new `marketplace/add` RPC should reuse the same underlying
marketplace-add flow as the CLI. This refactor lands that consolidation
first so the follow-up app-server PR can be mostly protocol and handler
wiring.

## Validation

- `cargo test -p codex-core marketplace_add`
- `cargo test -p codex-cli marketplace_cmd`
- `just fix -p codex-core`
- `just fix -p codex-cli`
- `just fmt`

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -331,6 +331,10 @@ client_request_definitions! {
         params: v2::SkillsListParams,
         response: v2::SkillsListResponse,
     },
+    MarketplaceAdd => "marketplace/add" {
+        params: v2::MarketplaceAddParams,
+        response: v2::MarketplaceAddResponse,
+    },
     PluginList => "plugin/list" {
         params: v2::PluginListParams,
         response: v2::PluginListResponse,