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/core/tests/suite/request_compression.rs

@@ -40,6 +40,7 @@ async fn request_body_is_zstd_compressed_for_codex_backend_when_enabled() -> any
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "compress me".into(),
                 text_elements: Vec::new(),
@@ -88,6 +89,7 @@ async fn request_body_is_not_compressed_for_api_key_auth_even_when_enabled() ->
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "do not compress".into(),
                 text_elements: Vec::new(),