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.

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

codex-rs/app-server/tests/suite/v2/skills_list.rs

@@ -324,6 +324,7 @@ async fn skills_changed_notification_is_emitted_after_skill_change() -> Result<(
             ephemeral: None,
             session_start_source: None,
             dynamic_tools: None,
+            environments: None,
             mock_experimental_field: None,
             experimental_raw_events: false,
             persist_extended_history: false,