Flaky TestSnapshotSummary#testFileSizeSummaryWithDVs caused by generateContentLength returning 0

[lilei1128]

TestSnapshotSummary#testFileSizeSummaryWithDVs is a flaky test that fails intermittently in CI.

FileGenerationUtil.generateContentLength() uses random().nextInt(10_000), which can return 0 with a probability of 1/10,000.

When a DV's contentSizeInBytes is 0:

1. addedSize accumulates to 0 in UpdateMetrics.addedFile()
2. SnapshotSummary.addTo() skips writing the "added-files-size" field due to the guard setIf(addedSize > 0, builder, ADDED_FILE_SIZE_PROP, addedSize)
3. The snapshot summary ends up with 18 fields instead of the expected 19
4. assertThat(summary2).hasSize(19) fails

Failure Output

Fix

Change generateContentLength() in FileGenerationUtil to guarantee a positive value: