Broadcasts Stats: Add sent_after and sent_before params

[n7studios]

Summary

Adds support for the sent_after and sent_before parameters on the get_broadcasts_stats method, which was added to the API on May 8th 2026:
https://developers.kit.com/changelog#may-8-2026-%E2%80%94-broadcast-stats

Testing

• testGetBroadcastsStatsWithSentAfter: Test that get_broadcasts_stats() returns the expected data when a valid sent_after date is specified.
• testGetBroadcastsStatsWithSentAfterNow: Test that get_broadcasts_stats() returns no broadcasts when a sent_after date is specified that is after all broadcasts.
• testGetBroadcastsStatsWithSentBefore: Test that get_broadcasts_stats() returns the expected data when a valid sent_before date is specified.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)