## Why
Remote environment registration can return an explicit `503` `registration_conflict` after the registry finishes its own recovery attempts. Retrying ambiguous failures is unsafe because a timed-out request may still have replaced a newer registration.
## What changed
- Retry only explicit `registration_conflict` responses with jittered backoff, both during initial registration and after a rejected relay connection.
- Apply the existing connection timeout to the complete registration attempt, including response-body diagnostics.
- Keep retry waits and in-flight requests cancellable while preserving the Noise identity and initialized session across successful recovery.
## Testing
Added unit coverage for retry eligibility and timeout handling, plus relay tests for identity and session preservation and prompt shutdown during registration.
GitOrigin-RevId: 374e6fdac4755f377cbea9400c76a228b790945a
e325e3acd9