[ECO-5551] Use endpoint as default connection option (ADR-119)

[maratal]

Closes #2098

Summary by CodeRabbit

• New Features

  • Endpoint-centric configuration with a domain selector and configurable connectivity check URL.

• Deprecations

  • environment, restHost and realtimeHost marked deprecated — migrate to endpoint.

• Removals

  • Legacy fallback-host API/class removed; fallback domains now come from the domain selector.

• Bug Fixes / Improvements

  • Improved domain/fallback resolution, URL derivation, connectivity checks, WebSocket host handling, and standardized defaults.

• Tests

  • Added domain-selector tests and updated suites to endpoint/primary-domain semantics.

• Chores

  • CI/test sandbox endpoints and scripts updated to non-production URLs.

_{✏️ Tip: You can customize this high-level summary in your review settings.}