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/app-server/src/codex_message_processor.rs

@@ -318,6 +318,7 @@ use codex_protocol::protocol::ReviewTarget as CoreReviewTarget;
 use codex_protocol::protocol::RolloutItem;
 use codex_protocol::protocol::SessionConfiguredEvent;
 use codex_protocol::protocol::SessionMetaLine;
+use codex_protocol::protocol::TurnEnvironmentSelection;
 use codex_protocol::protocol::USER_MESSAGE_BEGIN;
 use codex_protocol::protocol::W3cTraceContext;
 use codex_protocol::user_input::MAX_USER_INPUT_TEXT_CHARS;
@@ -7162,6 +7163,15 @@ impl CodexMessageProcessor {
         let collaboration_mode = params.collaboration_mode.map(|mode| {
             self.normalize_turn_start_collaboration_mode(mode, collaboration_modes_config)
         });
+        let environments = params.environments.map(|environments| {
+            environments
+                .into_iter()
+                .map(|environment| TurnEnvironmentSelection {
+                    environment_id: environment.environment_id,
+                    cwd: environment.cwd,
+                })
+                .collect()
+        });
 
         // Map v2 input items to core input items.
         let mapped_items: Vec<CoreInputItem> = params
@@ -7213,6 +7223,7 @@ impl CodexMessageProcessor {
                 thread.as_ref(),
                 Op::UserInput {
                     items: mapped_items,
+                    environments,
                     final_output_json_schema: params.output_schema,
                     responsesapi_client_metadata: params.responsesapi_client_metadata,
                 },