Add a partialResults flag in Response metadata

It would be good to let the broker add a partial result flag, case I can think when this flag is set to true:
1. Any exceptions during segment processing or timeout
2. Routing table doesn't contain full segments, e.g. all replicas are down for a segment in idealStates.

Then client-side can leverage this flag to decide the behavior like retry or just surface partial results.

Start by tracing the broker's Response metadata and the segment-processing timeout and exception paths, then inspect routing-table and idealStates handling for incomplete segments. Define how partialResults is set and exposed to clients, and add coverage for processing failures, timeouts, and unavailable replicas once the response contract is settled.

Written by the indexing model from the issue text.