test(auth): make the Windows inconclusive path in UserDataStorageMergeTest visible and observed (#894 follow-up)

[fluck-boss]

Follow-up from Claude's merge review of #894 (merged as 590a785). This did not block merge.

1. The Windows branch is reasoned, not observed. On Windows, the torn-read reader holds user_data.json open and can block Files.move. In that case the race test now treats the run as inconclusive (IS_WINDOWS && decodedForms.size <= 1). Since ci: run the Windows legs only for PRs into main #1631, dev PRs run only ubuntu, so this branch has never actually run in CI. It will first run on a PR into main. Check the first Windows run on a main-bound PR and confirm the branch behaves as intended.
2. The skip is silent. A Windows run that proved nothing passes with no trace in the output. Log a line (or collect a note and print it at the end) so "inconclusive on this leg" shows up in the test log.
3. Nit: the empty if branch around UserDataStorageMergeTest.kt:441-450 is comment-only. Invert the condition so it doesn't read as unfinished.