Add sticky environment API and thread state (#18897)

## Summary
- add sticky environment selections to app-server v2 thread/start and
turn/start request flow
- carry thread-level selections through core session/thread state
- add app-server coverage for sticky selections and turn overrides

## Stack
1. This PR: API and thread persistence
2. #18898: config.toml named environment loading
3. #18899: downstream tool/runtime consumers

## Validation
- Not run locally; split only.

---------

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

codex-rs/core/src/codex_delegate.rs

@@ -47,6 +47,7 @@ use crate::session::SUBMISSION_CHANNEL_CAPACITY;
 use crate::session::emit_subagent_session_started;
 use crate::session::session::Session;
 use crate::session::turn_context::TurnContext;
+use crate::session::turn_context::TurnEnvironment;
 use codex_login::AuthManager;
 use codex_models_manager::manager::ModelsManager;
 use codex_protocol::error::CodexErr;
@@ -73,7 +74,6 @@ pub(crate) async fn run_codex_thread_interactive(
 ) -> Result<Codex, CodexErr> {
     let (tx_sub, rx_sub) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
     let (tx_ops, rx_ops) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
-
     let CodexSpawnOk { codex, .. } = Box::pin(Codex::spawn(CodexSpawnArgs {
         config,
         auth_manager,
@@ -94,6 +94,11 @@ pub(crate) async fn run_codex_thread_interactive(
         inherited_exec_policy: Some(Arc::clone(&parent_session.services.exec_policy)),
         parent_rollout_thread_trace: codex_rollout_trace::ThreadTraceContext::disabled(),
         parent_trace: None,
+        environments: parent_ctx
+            .environments
+            .iter()
+            .map(TurnEnvironment::selection)
+            .collect(),
         analytics_events_client: Some(parent_session.services.analytics_events_client.clone()),
         thread_store: Arc::clone(&parent_session.services.thread_store),
     }))