test: cover time zone changes after mount

[gpbl]

Changing the timeZone prop after mount intentionally reinitializes an uncontrolled displayed month. Without lifecycle coverage, removing that synchronization can leave the calendar rendering a month from the previous zone.

What Changed

• Add a DayPicker component test using a fixed instant that falls in December in New York and January in Berlin.
• Assert the initial month and the month displayed after rerender as separate behaviors.

Review Notes

• The test uses explicit IANA zones and does not depend on the process time zone.
• The test passes on main and fails on the exact head of fix: changing startMonth and endMonth makes the calendar disappear #3005, where the calendar remains on December after the Berlin rerender.
• This is test-only, so no changeset is included.

[changeset-bot]

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Latest commit: 3f7a5da

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