[1/8] Add MCP server environment config (#18085)

## 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
```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
│
o  #18086 [2/8] Support piped stdin in exec process API
│
@  #18085 [1/8] Add MCP server environment config
│
o  main
```

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

codex-rs/config/src/mcp_edit.rs

@@ -174,6 +174,9 @@ fn serialize_mcp_server(config: &McpServerConfig) -> TomlItem {
     if !config.enabled {
         entry["enabled"] = value(false);
     }
+    if let Some(environment) = &config.experimental_environment {
+        entry["experimental_environment"] = value(environment.clone());
+    }
     if config.required {
         entry["required"] = value(true);
     }