Bound the node registry reads used by the cluster health checks

## Impact

Two checks in the System Overview health panel, **Certificates** and **OpenSearch Version** (both under Search Cluster), can be blocked on MongoDB reads that the health plugin cannot put a time limit on.

When one of those reads is slow or never returns, the check reports `unknown` instead of a definite state. An `unknown` check is dropped from the panel's rollup unless everything beside it is also unknown, so the section above it can stay green while a real problem goes unreported.

## Why the plugin cannot fix this itself

Each check gets a fixed time budget, and the framework can only ignore a late answer, not stop the work behind it. So every check has to impose its own limit on the call it makes. Most MongoDB-backed checks do that with the driver's `withTimeout`, which bounds the whole operation including retries. These two cannot, because the collection is a private field rather than something the caller can reach.

## What to do

Give each service a bounded read that a caller on a deadline can use. Either a timeout-carrying overload alongside the existing method, or expose a bounded read the callers can drive. Keep it additive so existing callers are untouched.

## Dependencies

None. This touches `AbstractNodeService` and `DataNodeMetadataServiceImpl` only, and conflicts with nothing currently in flight. It is deliberately split out of #26886, whose `ClusterAdapter` half is blocked on #26885 and touches the three search client classes instead.

## Consumers

Tracked on the Enterprise side in Graylog2/graylog-plugin-enterprise#15110, under "Bounded node registry reads".

Start by reading AbstractNodeService and DataNodeMetadataServiceImpl, then compare nearby MongoDB-backed checks that use the driver's withTimeout. Add an additive bounded-read API for each service without changing existing callers; done means the Certificates and OpenSearch Version health checks can enforce their deadlines without unbounded registry reads.

Written by the indexing model from the issue text.