Explain why the diagnostics spec's SET LOCAL needs a transaction

What does this MR do and why?

Adds a two-line comment above the SET LOCAL call in the real-database example of diagnostics_rake_spec.rb. SET LOCAL lasts only for the current transaction, and this example gets its transaction from the transactional fixtures that spec_helper turns on. A :delete example turns that wrapper off, so the call would do nothing there. The comment says so, which keeps a later :delete tag from silently dropping the timeout.

@Quintasan asked for it in !262569 (comment 4094649481), and I said I would follow up in !262569 (comment 4112706602).

References

Follows up !262569 (merged)

Screenshots or screen recordings

Not applicable, comment-only change.

How to set up and validate locally

Nothing to run: no code changes. The diff is two comment lines.

An AI coding assistant wrote this change.