[WIP][Shannon][Caching] Implement intelligent session caching based on actual session expiration block height

[commoddity]

🌿 Summary

Enable intelligent session caching in Shannon SDK gateways by aligning session refresh logic with blockchain block height instead of time-based TTLs.

🌱 Primary Changes:

• Implement block-aware session monitoring using SessionEndBlockHeight with support for zero-downtime cache swaps.
• Transition from time-based TTL to intelligent block-based session refresh lifecycle.
• Add background sync system to pre-fetch and hot-swap sessions before expiration.
• Replace early-refresh strategy with a polling mechanism at 1-second intervals near block height boundaries.
• Restructure GatewayClientCache to support dynamic cache invalidation and background refresh.

🍃 Secondary changes:

• Refactor and document caching constants and system architecture.
• Simplify and consolidate configuration logic; remove TTL and early-refresh toggles.
• Improve comments/docstrings across client packages for readability and maintainability.
• Refactor and modularize sessionRefreshState, including internal helper methods.
• Update GRPC client and associated error handling to reflect new session fetching strategy.

🛠️ Type of change

Select one or more from the following:

• New feature, functionality or library

🤯 Sanity Checklist

• I have updated the GitHub Issue 'assignees', 'reviewers', 'labels', 'project', 'iteration' and 'milestone'
• I added TODOs where applicable