[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
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o  #18089 [6/8] Wire remote MCP stdio through executor
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o  #18088 [5/8] Add executor process transport for MCP stdio
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o  #18087 [4/8] Abstract MCP stdio server launching
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o  #18020 [3/8] Add pushed exec process events
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o  #18086 [2/8] Support piped stdin in exec process API
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@  #18085 [1/8] Add MCP server environment config
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o  main