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/remote_models.rs

@@ -172,6 +172,7 @@ async fn remote_models_config_context_window_override_clamps_to_max_context_wind
             service_tier: None,
             collaboration_mode: None,
             personality: None,
+            environments: None,
         })
         .await?;
 
@@ -248,6 +249,7 @@ async fn remote_models_config_override_above_max_uses_max_context_window() -> Re
             service_tier: None,
             collaboration_mode: None,
             personality: None,
+            environments: None,
         })
         .await?;
 
@@ -323,6 +325,7 @@ async fn remote_models_use_context_window_when_config_override_is_absent() -> Re
             service_tier: None,
             collaboration_mode: None,
             personality: None,
+            environments: None,
         })
         .await?;
 
@@ -411,6 +414,7 @@ async fn remote_models_long_model_slug_is_sent_with_high_reasoning() -> Result<(
             service_tier: None,
             collaboration_mode: None,
             personality: None,
+            environments: None,
         })
         .await?;
 
@@ -474,6 +478,7 @@ async fn namespaced_model_slug_uses_catalog_metadata_without_fallback_warning()
             service_tier: None,
             collaboration_mode: None,
             personality: None,
+            environments: None,
         })
         .await?;
 
@@ -636,6 +641,7 @@ async fn remote_models_remote_model_uses_unified_exec() -> Result<()> {
             service_tier: None,
             collaboration_mode: None,
             personality: None,
+            environments: None,
         })
         .await?;
 
@@ -861,6 +867,7 @@ async fn remote_models_apply_remote_base_instructions() -> Result<()> {
             service_tier: None,
             collaboration_mode: None,
             personality: None,
+            environments: None,
         })
         .await?;