fix: address code review findings

internal/yandex/client.go

@@ -15,6 +15,9 @@ import (
 	"trip-planner/internal/metrics"
 )
 
+// rng is a seeded random number generator for jitter calculations.
+var rng = rand.New(rand.NewSource(time.Now().UnixNano()))
+
 // Client represents a Yandex Schedules API client with rate limiting,
 // circuit breaking, and retry capabilities.
 type Client struct {
@@ -421,6 +424,5 @@ func applyJitter(backoff time.Duration) time.Duration {
 }
 
 func randFloat64() float64 {
-	// Use math/rand with a seed based on function call index for variability
-	return rand.Float64()
+	return rng.Float64()
 }