Add explicit remote executor connection refresh (#40710)

## Why

Planned executor replacement needs a fresh session without waiting for the old
session's transient-disconnect recovery to finish.

## What changed

- Add `Environment::refresh_connection` for remote Noise registry-backed
  environments. It performs a fresh registry lookup, reuses a healthy session
  when the executor identity is unchanged, and connects to a replacement when
  it has changed.
- Retire superseded sessions and connection attempts so they cannot publish
  stale state, accept late RPC results, or replay outstanding work.
- Preserve the existing environment and filesystem handles while replacing the
  underlying client, and require a live status probe before refresh succeeds.

## Testing

Add coverage for replacement and session reuse, recovery and connection races,
lookup and handshake failures, handle preservation, and late RPC responses.

GitOrigin-RevId: f1d11208cbfe8af8feb25f6b6b8100da82169a99