[5/6] Wire executor-backed MCP stdio (#18212)

## Summary
- Add the executor-backed RMCP stdio transport.
- Wire MCP stdio placement through the executor environment config.
- Cover local and executor-backed stdio paths with the existing MCP test
helpers.

## Stack
```text
o  #18027 [6/6] Fail exec client operations after disconnect
│
@  #18212 [5/6] Wire executor-backed MCP stdio
│
o  #18087 [4/6] Abstract MCP stdio server launching
│
o  #18020 [3/6] Add pushed exec process events
│
o  #18086 [2/6] Support piped stdin in exec process API
│
o  #18085 [1/6] Add MCP server environment config
│
o  main
```

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/Cargo.lock

@@ -2414,6 +2414,7 @@ dependencies = [
  "async-channel",
  "codex-async-utils",
  "codex-config",
+ "codex-exec-server",
  "codex-login",
  "codex-otel",
  "codex-plugin",
@@ -2715,6 +2716,7 @@ dependencies = [
  "axum",
  "codex-client",
  "codex-config",
+ "codex-exec-server",
  "codex-keyring-store",
  "codex-protocol",
  "codex-utils-cargo-bin",