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.

test/mock-upstream/Dockerfile

@@ -0,0 +1,13 @@
+FROM golang:1.24-alpine AS builder
+
+WORKDIR /app
+
+COPY main.go .
+
+RUN go mod init mockupstream && go mod tidy && go build -o mock-upstream main.go
+
+FROM alpine:latest
+WORKDIR /app
+COPY --from=builder /app/mock-upstream .
+EXPOSE 8082
+CMD ["./mock-upstream"]