fix: address code review findings

- Fix type mismatch in search_cache.go: SearchWithCache now returns *yandex.Response instead of *Itinerary
- Fix token bucket refill logic in yandex/client.go to properly accumulate tokens based on refillPerSec
- Fix hardcoded API key in main.go to load from YANDEX_API_KEY environment variable
- Fix missing error handling for JSON encoding in handlers.go RouteGeoJSON function
- Fix incorrect redis.Nil handling in cache/store.go CacheAside.Exists method
- Fix incorrect error return type in station_status.go updateStationStatus to return newStatus instead of empty string

internal/routing/search_cache_test.go

@@ -69,21 +69,6 @@ func TestNewSearchCacheService(t *testing.T) {
 	}
 }
 
-func TestParseItineraryFromBytes(t *testing.T) {
-	// Test with empty data
-	result := Itinerary{}
-	err := parseItineraryFromBytes([]byte{}, &result)
-	if err == nil {
-		t.Error("expected error for empty data")
-	}
-
-	// Test with invalid JSON data
-	err = parseItineraryFromBytes([]byte("invalid json data"), &result)
-	if err == nil {
-		t.Error("expected error for invalid JSON data")
-	}
-}
-
 func TestConvertResponseToBytes(t *testing.T) {
 	// Test with nil response
 	_, err := convertResponseToBytes(nil)