## Why
Remote environment provisioning can finish after a thread starts, before an
exec-server WebSocket URL is available.
## What changed
- Add `EnvironmentManager::register_pending_environment` and a one-shot
`PendingEnvironmentRegistration` handle that resolves to either a validated
WebSocket URL or a terminal provisioning error.
- Let lazy remote exec-server clients wait for that result, while preserving
reconnection behavior after a successful registration.
- Keep replacement registrations isolated so completing an older handle does
not resolve the current environment with the same ID.
## Testing
Add coverage for successful connection and reconnection, provisioning and
dropped-registration failures, invalid URLs, replacement isolation, and the
deferred-executor startup flow.
GitOrigin-RevId: 5c05be2b72291b77a1f71176d7075b1ad63332a5
pakrym-oai
4472698728