## Why
Concurrent credential refreshes could restore a removed environment by overwriting the selection list with stale state. Unpolled snapshot futures could also keep pending configuration alive after an environment was removed.
## What changed
- Protect environment selections with a mutex and update them in place to serialize selection changes, configuration refreshes, and connection listener setup.
- Capture only selection metadata and resolution futures in snapshots so they do not retain pending configuration senders.
## Testing
Add regression tests for removal during credential refresh and cancellation while an unpolled snapshot is held. Strengthen the pending attachment integration test to check that `wait_for_environment` returns the expected environment ID and `ready` status.
GitOrigin-RevId: c6f933e8d3031b05335ddcfed00ab400aa5bad2a
sayan-oai
62ea6d41e8