MBTI-PDF-RESULT-SNAPSHOT-PARITY-REALIGN-08: bump snapshot PDF surface to v4

[fermatmind]

What changed

• Bumped backend MBTI result-page snapshot surface headers/cache identity to mbti.result_page_snapshot.v4.
• Updated API error normalization to recognize the v4 result-page snapshot surface.
• Updated focused PDF parity tests to expect v4 and verify old v3 artifacts are not reused.

Why

The previous v3 surface has been semantically polluted by summary-shell artifacts. v4 provides a clean cache and contract boundary for the corrected result-page snapshot rendering path.

Validation

• php artisan test --filter=AttemptPublicReportPdfParityTest
• vendor/bin/pint --dirty
• git diff --check

Intentionally deferred

• No deployment or server configuration changes.
• No production artifact/cache cleanup.
• No CMS, DB, queue, search, or indexing changes.
• No mPDF fallback changes.