[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/suite/v2/marketplace_add.rs

@@ -0,0 +1,40 @@
+use anyhow::Result;
+use app_test_support::McpProcess;
+use codex_app_server_protocol::MarketplaceAddParams;
+use codex_app_server_protocol::RequestId;
+use tempfile::TempDir;
+use tokio::time::Duration;
+use tokio::time::timeout;
+
+const DEFAULT_TIMEOUT: Duration = Duration::from_secs(10);
+
+#[tokio::test]
+async fn marketplace_add_rejects_local_directory_source() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_TIMEOUT, mcp.initialize()).await??;
+
+    let request_id = mcp
+        .send_marketplace_add_request(MarketplaceAddParams {
+            source: "./marketplace".to_string(),
+            ref_name: None,
+            sparse_paths: None,
+        })
+        .await?;
+
+    let err = timeout(
+        DEFAULT_TIMEOUT,
+        mcp.read_stream_until_error_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+
+    assert_eq!(err.error.code, -32600);
+    assert!(
+        err.error.message.contains(
+            "local marketplace sources are not supported yet; use an HTTP(S) Git URL, SSH Git URL, or GitHub owner/repo"
+        ),
+        "unexpected error: {}",
+        err.error.message
+    );
+    Ok(())
+}

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

@@ -15,6 +15,7 @@ mod experimental_api;
 mod experimental_feature_list;
 mod fs;
 mod initialize;
+mod marketplace_add;
 mod mcp_resource;
 mod mcp_server_elicitation;
 mod mcp_server_status;