Fix AttemptsHelpers.GetHost implementation

[sacOO7]

• The current approach for retrieving the next fallback host is overly complex and deviates from the spec.
• A temporary fix has been applied to resolve the primary host → [ECO-5646] Fix behaviour when internet connection is not available #1320
• However, the existing implementation needs to be refactored to properly handle both primary and fallback hosts.
• Reference implementation aligned with the spec → https://github.com/ably/ably-go/pull/657/files

┆Issue is synchronized with this Jira Task by Unito

[sacOO7]

More of related PRs/commits

1. Generate env fallbacks ably-go#268
2. CanFallback impl -> https://sdk.ably.com/builds/ably/specification/main/features/#RSC15l