test: Add tests for Zstd compression level configuration

Add test cases to verify:
- Default compression level (3)
- Custom compression level setting

test/test_out_s3.rb

@@ -474,7 +474,11 @@ def test_write_with_zstd
 
     expected_s3path = "log/events/ts=20110102-13/events_0-#{Socket.gethostname}.zst"
 
-    setup_s3_object_mocks(s3_local_file_path: s3_local_file_path, s3path: expected_s3path)
+    setup_s3_object_mocks(
+      s3_local_file_path: s3_local_file_path,
+      s3path: expected_s3path,
+      content_type: "application/x-zst"
+    )
 
     config = CONFIG_TIME_SLICE + "\nstore_as zstd\n"
     d = create_time_sliced_driver(config)