[2/8] Support piped stdin in exec process API

[aibrahim-oai]

Summary

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

Stack

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

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve backward-compatible default for stdin mode

Make stdin optional at the wire level. As added, ExecParams now requires stdin, so older process/start clients that omit it fail deserialization with invalid params instead of starting a process. There are existing JSON callers that don't send this field (for example in websocket process tests), so this change introduces a protocol break. Add a serde default of Closed.

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