[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/core/tests/suite/truncation.rs

@@ -375,6 +375,7 @@ async fn mcp_tool_call_output_exceeds_limit_truncated_for_model() -> Result<()>
                     env_vars: Vec::new(),
                     cwd: None,
                 },
+                experimental_environment: None,
                 enabled: true,
                 required: false,
                 supports_parallel_tool_calls: false,
@@ -472,6 +473,7 @@ async fn mcp_image_output_preserves_image_and_no_text_summary() -> Result<()> {
                     env_vars: Vec::new(),
                     cwd: None,
                 },
+                experimental_environment: None,
                 enabled: true,
                 required: false,
                 supports_parallel_tool_calls: false,
@@ -752,6 +754,7 @@ async fn mcp_tool_call_output_not_truncated_with_custom_limit() -> Result<()> {
                     env_vars: Vec::new(),
                     cwd: None,
                 },
+                experimental_environment: None,
                 enabled: true,
                 required: false,
                 supports_parallel_tool_calls: false,