[2/8] Support piped stdin in exec process API (#18086)

## Summary
- Add an explicit stdin mode to process/start.
- Keep normal non-interactive exec stdin closed while allowing
pipe-backed processes.

## Stack
```text
o  #18027 [8/8] Fail exec client operations after disconnect
│
o  #18025 [7/8] Cover MCP stdio tests with executor placement
│
o  #18089 [6/8] Wire remote MCP stdio through executor
│
o  #18088 [5/8] Add executor process transport for MCP stdio
│
o  #18087 [4/8] Abstract MCP stdio server launching
│
o  #18020 [3/8] Add pushed exec process events
│
@  #18086 [2/8] Support piped stdin in exec process API
│
o  #18085 [1/8] Add MCP server environment config
│
o  main
```

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

codex-rs/exec-server/src/environment.rs

@@ -346,6 +346,7 @@ mod tests {
                 env_policy: None,
                 env: Default::default(),
                 tty: false,
+                pipe_stdin: false,
                 arg0: None,
             })
             .await