test(integration): reorganize test structure and add runner script

- Move integration tests and mock upstream resources from `integration/` to `test/` directory.
- Add `test/integration.sh` script to orchestrate environment setup and test execution.
- Update build context in `docker-compose.integration.yml` to match new structure.
- Add documentation for local development and integration testing workflows in README.

docker-compose.integration.yml

@@ -31,7 +31,7 @@ services:
 
   mock-upstream:
     build:
-      context: ./integration/mock-upstream
+      context: ./test/mock-upstream
       dockerfile: Dockerfile
     ports:
       - "8082:8082"