feat: implement Yandex /search method for on-demand edge expansion (Task 2)

- Add SearchRoutes method to yandex client for on-demand station pair searches
- Implement hub expansion via on-demand /search calls in lazy graph expansion
- Integrate cache key generation for search results (search:{from}:{to}:{date})
- Update expandFromStation and expandFromCityHub to use Yandex API
- Add NewGraphWithoutYandex constructor for testability
- Add tests for on-demand search integration and cache TTL policies

internal/yandex/client.go

@@ -233,8 +233,9 @@ type Segment struct {
 
 // Station represents a station in the API response.
 type Station struct {
-	Code  string `json:"code"`
-	Title string `json:"title"`
+	Code          string `json:"code"`
+	Title         string `json:"title"`
+	TransportType string `json:"transport_type"`
 	// Other fields can be added as needed
 }
 
@@ -273,6 +274,17 @@ func buildURL(path string, query map[string]string) string {
 	return u
 }
 
+// SearchRoutes searches for routes between two stations on a given date.
+// This is used for on-demand edge expansion in the lazy graph expansion strategy.
+func (c *Client) SearchRoutes(ctx context.Context, from, to, date string) (*Response, error) {
+	query := map[string]string{
+		"from": from,
+		"to":   to,
+		"date": date,
+	}
+	return c.Do(ctx, "GET", "/v3.0/search/", query)
+}
+
 // --- Token Bucket Rate Limitter ---
 
 func newTokenBucket(capacity, perSeconds int) *tokenBucket {