java: format spanner samples after spanner team confirms this doesn't impact public docs

@rahul2393 I am not saying that we shouldn't do this. My concern is that we have done it in this way probably because we had concerns with the formatting in the past. We should clarify with the docs team before we do this change is what I am suggesting. (Since samples was explicitly removed in https://github.com/googleapis/synthtool/pull/1295 maybe because formatting is reducing the snippet clarity or some other reason).

_Originally posted by @sakthivelmanii in https://github.com/googleapis/google-cloud-java/issues/13228#issuecomment-4490936514_

Review googleapis/google-cloud-java issue #13228 and googleapis/synthtool PR #1295 to understand why sample formatting was removed. Confirm with the docs team whether formatting affects snippet clarity; done means the concern is resolved and the scope of any safe change is clear.

Written by the indexing model from the issue text.