Stats: fix CSV export filenames for selected date ranges - #113340

Fixes STATS-420

Proposed Changes

• Prefer the custom date-range query (start_date / date) when building Stats CSV download filenames, so the filename matches the selected range instead of a single period unit around the start date.
• Omit the period segment for custom ranges (query.period is forced to day for the API and is not meaningful in the filename), e.g. …-posts-01/01/2026-08/06/2026.csv instead of …-posts-day-01/01/2026-08/06/2026.csv.
• Extract getStatsCsvFileName and add regression coverage for the STATS-420 cases (multi-month and single-month custom ranges).
• Drop the period and date segments from the Emails summary export filename via a new includeDates option, so it downloads as <site-slug>-emails.csv.

Why are these changes being made?

Exported report filenames did not reflect the selected date range. For a Jan 1–Aug 6 selection the file looked like a January-only month export, and for Jan 1–31 the filename used a day period with identical start/end dates. The CSV contents were correct; only the filename was wrong.

Custom date ranges also always forced query.period to day, which left a misleading day- prefix in every custom-range export even after the dates were fixed.

The Emails summary report has no date picker and its export is not date-scoped — it always returns the latest 30 emails — so a date range in that filename was misleading and is now omitted.

Testing Instructions

1. Open Jetpack Stats → Posts (or another summary module with CSV download).
2. Select a custom range spanning multiple months (e.g. Jan 1–Aug 6, 2026) and download CSV.
3. Confirm the filename includes the full selected range without a period segment (e.g. …-posts-01/01/2026-08/06/2026.csv), not …-posts-day-… or a single month.
4. Select a single-month custom range (e.g. Jan 1–31) and download CSV.
5. Confirm the filename uses that full range without a period segment, not …-day-01/01/2026-01/01/2026.csv.
6. Select a standard period without a custom range (e.g. month view) and confirm the filename still uses that period’s label and start/end (e.g. …-month-…).
7. Open Stats → Emails (/stats/day/emails/<site>) and download CSV. Confirm the filename is <site-slug>-emails.csv with no period or dates.
8. Optional: yarn test-client client/my-sites/stats/stats-download-csv/test/get-stats-csv-filename.js

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• For UI changes, have you tested the affected components in dark mode?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

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