Add turn-scoped environment selections (#18416)

## Summary
- add experimental turn/start.environments params for per-turn
environment id + cwd selections
- pass selections through core protocol ops and resolve them with
EnvironmentManager before TurnContext creation
- treat omitted selections as default behavior, empty selections as no
environment, and non-empty selections as first environment/cwd as the
turn primary

## Testing
- ran `just fmt`
- ran `just write-app-server-schema`
- not run: unit tests for this stacked PR

---------

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

codex-rs/mcp-server/src/codex_tool_runner.rs

@@ -108,6 +108,7 @@ pub async fn run_codex_tool_session(
     let submission = Submission {
         id: sub_id.clone(),
         op: Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: initial_prompt.clone(),
                 // MCP tool prompts are plain text with no UI element ranges.
@@ -156,6 +157,7 @@ pub async fn run_codex_tool_session_reply(
         .insert(request_id.clone(), thread_id);
     if let Err(e) = thread
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: prompt,
                 // MCP tool prompts are plain text with no UI element ranges.