Why

This is a follow-up to #15360. That change fixed the arg0 helper setup, but rmcp-client still coerced stdio transport environment values into UTF-8 Strings before program resolution and process spawn. If PATH or another inherited environment value contains non-UTF-8 bytes, that loses fidelity before it reaches which and Command.

What changed

• change create_env_for_mcp_server() to return HashMap<OsString, OsString> and read inherited values with std::env::var_os()
• change TransportRecipe::Stdio.env, RmcpClient::new_stdio_client(), and program_resolver::resolve() to keep stdio transport env values in OsString form within rmcp-client
• keep the codex-core config boundary stringly, but convert configured stdio env values to OsString once when constructing the transport
• update the rmcp-client stdio test fixtures and callers to use OsString env maps
• add a Unix regression test that verifies create_env_for_mcp_server() preserves a non-UTF-8 PATH

How to verify

• cargo test -p codex-rmcp-client
• cargo test -p codex-core mcp_connection_manager
• just argument-comment-lint

Targeted coverage in this change includes utils::tests::create_env_preserves_path_when_it_is_not_utf8, while the updated stdio transport path is exercised by the existing rmcp-client tests that construct RmcpClient::new_stdio_client().