feat(premium-analytics): expose initial_full_sync_finished on sync status response

[chihsuan]

Related to WOOA7S-1542. Backend half; the frontend consumer (#49267) stacks on top.

Proposed changes

Why: The analytics dashboard reads the initial-full-sync milestone (jetpack_premium_analytics_initial_full_sync_finished) to know when the first analytics sync has completed. Today Sync_Status_Tracker surfaces it only once, at page load, via JetpackScriptData — a static snapshot that can't reflect a sync finishing mid-session.

How: A rest_post_dispatch filter in Sync_Status_Tracker appends the milestone to Jetpack core's existing GET /jetpack/v4/sync/status response. No new endpoint and no Jetpack-core changes — the field rides along on the poll the dashboard already makes. Only the already-authorized, successful status payload is touched; other routes and error responses pass through untouched (no milestone leak).

What: GET /jetpack/v4/sync/status now includes an initial_full_sync_finished integer (0 until the analytics initial full sync completes, then the finish timestamp). Additive and standalone — no consumer in this PR (same split as the PHP/JS halves #49211 / #49267).

Does this pull request change what data or activity we track or use?

No.

Testing instructions

cd projects/packages/premium-analytics
composer phpunit   # 34 tests pass, incl. enrich_sync_status_response coverage

On a Jetpack-connected site with the package active:

• GET /jetpack/v4/sync/status now includes an initial_full_sync_finished integer.
• Confirm non-status routes (e.g. POST /jetpack/v4/sync/full-sync) and unauthorized/error responses are unaffected.

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/premium-analytics/src/Sync/class-sync-status-tracker.php	45/49 (91.84%)	2.36%	0 💚

Full summary · PHP report

[Copilot]

Pull request overview

This PR enhances the Premium Analytics sync milestone exposure by injecting the persisted initial_full_sync_finished timestamp into Jetpack core’s existing GET /jetpack/v4/sync/status REST response, enabling consumers (e.g., the analytics dashboard) to observe the milestone flipping mid-session via the existing polling mechanism.

Changes:

• Add a rest_post_dispatch filter in Sync_Status_Tracker to append initial_full_sync_finished to successful /jetpack/v4/sync/status responses.
• Extend PHPUnit coverage to validate enrichment behavior (correct route only, skips error responses, passthrough for non-REST responses).
• Add a changelog entry documenting the new REST response field.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
projects/packages/premium-analytics/src/Sync/class-sync-status-tracker.php	Adds REST response enrichment for /jetpack/v4/sync/status with the milestone timestamp.
projects/packages/premium-analytics/tests/php/Sync/Sync_Status_Tracker_Test.php	Adds unit tests covering the new REST enrichment behavior and hook registration.
projects/packages/premium-analytics/changelog/wooa7s-1542-expose-sync-milestone-live	Documents the new initial_full_sync_finished field on the sync status REST response.

[louwie17]

This looks good @chihsuan and tested well, nice work!