Cosmos: consolidate periodic account-metadata refresh loop into runtime-owned task

Follow-up tracking issue for the work in #4458 (closed without merging).

## Goal

Move the periodic database-account metadata refresh loop from per-`CosmosDriver` (one tokio task per driver) to per-`CosmosDriverRuntime` (single task iterating a registry of registered account endpoints). A runtime hosting N drivers should issue one fetch per registered account per interval, not N concurrent tasks. Originally introduced as the per-driver loop in #4407.

## References

- Original per-driver loop: #4407
- Closed PR with the consolidation work: #4458

Start by reviewing the closed consolidation work in #4458 and the original per-driver loop in #4407. Trace how the periodic refresh is owned by each CosmosDriver and how CosmosDriverRuntime registers account endpoints; done means one runtime-owned task fetches each registered account once per interval without one task per driver.

Written by the indexing model from the issue text.