Track executor registrations across connection refresh and recovery - #46555

…46555)

## Why

An executor can renew its registration while retaining its Noise identity. Connection refresh must distinguish registrations even when their keys match, and a stale registry lookup must not retire a session that recovery has already renewed.

## What changed

- Track the installed session's registration and expose it through `Environment::cached_executor_registration_id()` without connecting. Publish registration changes only when the new connection is installed.
- Require both registration and executor key to match before refresh reuses a session. Retry stale lookups and prevent recovery from installing a connection after retirement.
- Pin session recovery to the original Noise key while allowing registration renewal to preserve the session and running processes.
- Expose `CodexThread::active_turn_environment_selections()` so hosts can authorize steering against the active turn's selections, including starting and failed environments, independently of later settings updates.

## Testing

Add regression coverage for registration replacement, renewal with a running process, stale refresh lookups, missing sessions, and rejection of changed Noise keys. Extend active-turn tests to check selection snapshots across settings updates, deferred startup, and turn completion.

GitOrigin-RevId: 4395bc6c57f137691887e6585f52dcc5b958101c