[1/8] Add MCP server environment config

[aibrahim-oai]

Summary

• Add an MCP server environment setting with local as the default.
• Thread the default through config serialization, schema generation, and existing config fixtures.

Stack

o  #18027 [8/8] Fail exec client operations after disconnect
│
o  #18025 [7/8] Cover MCP stdio tests with executor placement
│
o  #17974 [6/8] Wire remote MCP stdio through executor
│
o  #17987 [5/8] Add executor process transport for MCP stdio
│
o  #17986 [4/8] Abstract MCP stdio server launching
│
o  #18020 [3/8] Add pushed exec process events
│
o  #17985 [2/8] Support piped stdin in exec process API
│
@  #17984 [1/8] Add MCP server environment config
│
o  main

[pakrym-oai]

make it a string.
experimental_environment too.

[aibrahim-oai]

Recreated as #18085 from an openai/codex head branch so internal CI uses the normal credentials path.