Harden Blaxel sandbox lifecycle

[mrubens]

Summary

• normalize Blaxel structured errors and apply bounded sequential retries across readiness-sensitive operations
• use deterministic external IDs with idempotent sandbox creation
• reuse preview resources across standby/resume instead of deleting and recreating them
• document the lifecycle behavior and add contract coverage

Validation

• full repository test suite
• pnpm lint
• pnpm check-types:fast
• pnpm knip
• focused compute-provider tests (23 passing)
• focused controller tests (2 passing)
• live local Blaxel smoke test through ngrok: fresh provision, retained standby, and same-machine resume

Live findings fixed

• sanitize deterministic external IDs to Blaxel-compatible alphanumeric/hyphen values
• fail deterministic 4xx lifecycle errors immediately while preserving retry behavior for 408/429 and retryable server/network failures

[roomote-roomote]

No code issues found. See task

_{Reviewed cd67b0b}