Monthly frequency should be set to previous month days

[zutigrm]

Feature Description

Originally rasied in bug bashing here: https://app.asana.com/1/8468971550303/project/1213555065417186/task/1213762140906173

Since early development, monthly frequency was called monthly but "rounded" to a 30 day report period in

site-kit-wp/includes/Core/Email_Reporting/Initiator_Task.php

Line 120 in d77d177

Email_Reporting_Settings::FREQUENCY_MONTHLY => 30,

I noticed that we use copy "28 days" in the frequency selector, which would also make sense as it reflect periods we offer on dashboard. We should probably fix this in the code to set period to 28.

As Ben suggested in the comment bellow, best would be to compute numbers of previous month and use that number.

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Acceptance criteria

• Monthly email-report reference dates are no longer based on a fixed 30-day window.
• For monthly runs, the report window should be the number of days in the previous calendar month in the site's timezone.
  • Eg. A "Last month" report sent in February should use the number of days in January (31); a report sent in May should use the number of days in April (30).
• The text used for Monthly period (eg. "Last 28 days" or "Last 30 days") should be changed to "Last month".

Note: sendDate is the last day of the previous month; startDate is the first day of that same month.

Implementation Brief

• Update includes/Core/Email_Reporting/Initiator_Task.php:
  • In build_reference_dates(), replace hardcoded monthly 30 with a computed value derived from the monthly send date:
    • Keep existing scheduled_date and send_date logic.
    • For monthly frequency, set period_days = (int) $send_date->format( 't' ).
    • Keep weekly (7) and quarterly (90) as-is.
• Update assets/js/components/email-reporting/FrequencySelector.js:
  • Replace monthly period copy (Last 28 days) with non-fixed wording (e.g. Last month) to align with variable month length and avoid misleading static text.

Test Coverage

• Update tests/phpunit/integration/Core/Email_Reporting/Initiator_TaskTest.php:
  • Add/adjust monthly cases to verify computed period length for different months (28, 29, 30, 31-day outcomes—eg. account for leap years, etc.).
  • Assert monthly startDate/sendDate map to first/last day of previous month for a month-boundary scheduled timestamp.
  • Keep weekly/quarterly assertions unchanged.
• Update any failing snapshots.

QA Brief

Changelog entry

[tofumatt]

I've adjusted the ACs to be a bit more explicit (including mentioning the change in text from "Last 28 days" to "Last month", which is important to this issue but not mentioned in the ACs, only the IB).

ACs and IB look good though, moving to the backlog 👍🏻