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/plugins.rs

@@ -192,6 +192,7 @@ async fn capability_sections_render_in_developer_message_in_order() -> Result<()
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![codex_protocol::user_input::UserInput::Text {
                 text: "hello".into(),
                 text_elements: Vec::new(),
@@ -268,6 +269,7 @@ async fn explicit_plugin_mentions_inject_plugin_guidance() -> Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![codex_protocol::user_input::UserInput::Mention {
                 name: "sample".into(),
                 path: format!("plugin://{SAMPLE_PLUGIN_CONFIG_NAME}"),
@@ -348,6 +350,7 @@ async fn explicit_plugin_mentions_track_plugin_used_analytics() -> Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![codex_protocol::user_input::UserInput::Mention {
                 name: "sample".into(),
                 path: format!("plugin://{SAMPLE_PLUGIN_CONFIG_NAME}"),