[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);
     }

codex-rs/config/src/mcp_edit_tests.rs

@@ -22,6 +22,7 @@ async fn replace_mcp_servers_serializes_per_tool_approval_overrides() -> anyhow:
                 env_vars: Vec::new(),
                 cwd: None,
             },
+            experimental_environment: None,
             enabled: true,
             required: false,
             supports_parallel_tool_calls: true,

codex-rs/config/src/mcp_types.rs

@@ -61,6 +61,10 @@ pub struct McpServerConfig {
     #[serde(flatten)]
     pub transport: McpServerTransportConfig,
 
+    /// Experimental environment selector for where Codex should start this MCP server.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub experimental_environment: Option<String>,
+
     /// When `false`, Codex skips initializing this MCP server.
     #[serde(default = "default_enabled")]
     pub enabled: bool,
@@ -139,6 +143,8 @@ pub struct RawMcpServerConfig {
 
     // shared
     #[serde(default)]
+    pub experimental_environment: Option<String>,
+    #[serde(default)]
     pub startup_timeout_sec: Option<f64>,
     #[serde(default)]
     pub startup_timeout_ms: Option<u64>,
@@ -181,6 +187,7 @@ impl TryFrom<RawMcpServerConfig> for McpServerConfig {
             url,
             bearer_token,
             bearer_token_env_var,
+            experimental_environment,
             startup_timeout_sec,
             startup_timeout_ms,
             tool_timeout_sec,
@@ -246,6 +253,7 @@ impl TryFrom<RawMcpServerConfig> for McpServerConfig {
 
         Ok(Self {
             transport,
+            experimental_environment,
             startup_timeout_sec,
             tool_timeout_sec,
             enabled: enabled.unwrap_or_else(default_enabled),

codex-rs/config/src/mcp_types_tests.rs

@@ -297,6 +297,7 @@ fn deserialize_ignores_unknown_server_fields() {
                 env_vars: Vec::new(),
                 cwd: None,
             },
+            experimental_environment: None,
             enabled: true,
             required: false,
             supports_parallel_tool_calls: false,