test(models): build edit scope boundaries in the time zone under test

Problem

• The tests pass a fixed zone, built from the server's current UTC offset, into CanEditTransactionByTransactionTime.
• They build the expected boundary in time.Local.
• Where a DST transition sits between that boundary and today, the two differ by an hour and the assertion fails.
• A runner in UTC never reports this.

Measurement

• Which test fails depends on where the transition falls. Santiago moved its clocks on 6 September, inside the current month, so there the month boundary misses rather than the year boundary.
• Zones without daylight saving time pass either way.
• -count=1 matters: without it the cached result of an earlier run is reported for every subsequent zone, because the time zone is not part of the cache key.

Rejected alternative

• Setting timezone to time.Local makes the tests pass just as well, measured in UTC, Europe/Berlin, America/Santiago and Pacific/Chatham.
• It changes what is under test. GetClientTimezone() returns a fixed zone whenever the request carries only X-Timezone-Offset and no X-Timezone-Name, and these tests are the ones covering that path.

Environment