fix: keep rmcp-client env vars as OsString (#15363)

## 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 `String`s 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()`.

codex-rs/rmcp-client/tests/process_group_cleanup.rs

@@ -73,8 +73,8 @@ async fn drop_kills_wrapper_process_group() -> Result<()> {
             ),
         ],
         Some(HashMap::from([(
-            "CHILD_PID_FILE".to_string(),
-            child_pid_file_str,
+            OsString::from("CHILD_PID_FILE"),
+            OsString::from(child_pid_file_str),
         )])),
         &[],
         None,