fix: pin requery to preferred replica (#49826)

issue: #49742

### What changed
- Propagate preferred shard leader hints for search requery workloads.
- Add shard-level metrics for preferred node hit, miss, unavailable, and
rejected results.
- Keep preferred-node selection outside the balancer path so it does not
affect balancer state.
- Preserve fallback to other serviceable replicas when the preferred
node is unavailable or excluded.

### Tests
- go test ./internal/proxy/shardclient

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Signed-off-by: chyezh <chyezh@outlook.com>