refactor: run Spark tests fully in Docker with bridge network

Better approach than mixing host and container networks.

Changes to docker-compose.yml:
- Remove 'network_mode: host' from spark-tests container
- Add spark-tests to seaweedfs-spark bridge network
- Update SEAWEEDFS_FILER_HOST from 'localhost' to 'seaweedfs-filer'
- Add depends_on to ensure services are healthy before tests
- Update volume publicUrl from 'localhost:8080' to 'seaweedfs-volume:8080'

Changes to workflow:
- Remove separate build and test steps
- Run tests via 'docker compose up spark-tests'
- Use --abort-on-container-exit and --exit-code-from for proper exit codes
- Simpler: one step instead of two

Benefits:
✓ All components use Docker DNS (seaweedfs-master, seaweedfs-volume, seaweedfs-filer)
✓ No host/container network split or DNS resolution issues
✓ Consistent with how other SeaweedFS integration tests work
✓ Tests are fully containerized and reproducible
✓ Volume server accessible via seaweedfs-volume:8080 for all clients
✓ Automatic volume creation works (master can reach volume via gRPC)
✓ Data writes work (Spark can reach volume via Docker network)

This matches the architecture of other integration tests and is cleaner.

.github/workflows/spark-integration-tests.yml

@@ -185,23 +185,11 @@ jobs:
         curl -s http://localhost:9333/dir/status | jq '.' || echo "jq not available"
         echo "✓ Volume server registered, volumes will be created on-demand during tests"
 
-    - name: Build Spark integration tests
+    - name: Run Spark integration tests in Docker
       working-directory: test/java/spark
       run: |
-        echo "Building Spark integration tests..."
-        mvn clean package -DskipTests
-        echo "✓ Build completed"
-
-    - name: Run Spark integration tests
-      working-directory: test/java/spark
-      env:
-        SEAWEEDFS_TEST_ENABLED: true
-        SEAWEEDFS_FILER_HOST: localhost
-        SEAWEEDFS_FILER_PORT: 8888
-        SEAWEEDFS_FILER_GRPC_PORT: 18888
-      run: |
-        echo "Running Spark integration tests..."
-        mvn test -B
+        echo "Running Spark integration tests in Docker container..."
+        docker compose up --abort-on-container-exit --exit-code-from spark-tests spark-tests
         echo "✓ Tests completed"
 
     - name: Upload test results