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>
starr-openai