feat: Implement on-demand /search integration in lazy graph expansion

- Integrate on-demand Yandex /search calls in FindRoute when lazy expansion + synthetic fallback fails
- Add real route segments from API response as edges, then retry BFS search
- Integrate existing cache key generation and TTL policies (SearchNearTermTTL: 3h, SearchFarTermTTL: 7d)
- Write tests: TestSearchRoutes_onDemand, TestSearchRoutes_onDemandVerifyIntegration, TestFindRouteWithSyntheticFallback
- All tests pass before task 5 (transfer depth limiting)

docs/plans/2026-08-15-full-implementation.md

@@ -71,11 +71,11 @@ Implement the complete multimodal trip planning service as specified in `docs/sp
 - [x] **Write tests:** TestResetCircuitBreaker
 - [x] Run tests - must pass before task 4
 
-### Task 4: Implement on-demand /search integration [ ]
-- [ ] Integrate on-demand `/search` calls in lazy graph expansion
-- [ ] Implement cache key generation and TTL policies
-- [ ] Write tests: TestSearchRoutes_onDemand with circuit breaker reset
-- [ ] Run tests - must pass before task 5
+### Task 4: Implement on-demand /search integration [x]
+- [x] Integrate on-demand `/search` calls in lazy graph expansion
+- [x] Implement cache key generation and TTL policies
+- [x] Write tests: TestSearchRoutes_onDemand with circuit breaker reset
+- [x] Run tests - must pass before task 5
 
 ### Task 5: Transfer depth limiting [ ]
 - [ ] Implement depth limiting in BFS/Dijkstra (max 4-5 transfers)