Add static shard picker

Author: ofekshenawa

internal/routing/shard_picker.go

@@ -11,6 +11,22 @@ type ShardPicker interface {
 	Next(total int) int // returns an index in [0,total)
 }
 
+// StaticShardPicker always returns the same shard index.
+type StaticShardPicker struct {
+	index int
+}
+
+func NewStaticShardPicker(index int) *StaticShardPicker {
+	return &StaticShardPicker{index: index}
+}
+
+func (p *StaticShardPicker) Next(total int) int {
+	if total == 0 || p.index >= total {
+		return 0
+	}
+	return p.index
+}
+
 /*───────────────────────────────
    Round-robin (default)
 ────────────────────────────────*/