[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/tests/common/mcp_process.rs

@@ -47,6 +47,7 @@ use codex_app_server_protocol::JSONRPCRequest;
 use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::ListMcpServerStatusParams;
 use codex_app_server_protocol::LoginAccountParams;
+use codex_app_server_protocol::MarketplaceAddParams;
 use codex_app_server_protocol::McpResourceReadParams;
 use codex_app_server_protocol::McpServerToolCallParams;
 use codex_app_server_protocol::MockExperimentalMethodParams;
@@ -514,6 +515,15 @@ impl McpProcess {
         self.send_request("skills/list", params).await
     }
 
+    /// Send a `marketplace/add` JSON-RPC request.
+    pub async fn send_marketplace_add_request(
+        &mut self,
+        params: MarketplaceAddParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("marketplace/add", params).await
+    }
+
     /// Send a `plugin/install` JSON-RPC request.
     pub async fn send_plugin_install_request(
         &mut self,