test: lock oversized PDF rejection policy branches

[Frankie-Xu]

Related to #121.

Summary

• Add worker unit coverage for oversized_pdf_policy.py. That module is the billing/admission gate for oversized PDFs and previously had no direct tests.
• Test-only. No production code, copy, or policy behavior change.
• Uses mocked MAX_PDF_PAGE_LIMIT / OVERSIZED_PDF_SHARD_ENABLED / OVERSIZED_PDF_SOFT_LIMIT (no real large PDFs).

Covered branches:

• non-PDF files are not rejected
• PDFs at or below MAX_PDF_PAGE_LIMIT are not rejected
• OVERSIZED_PDF_SHARD_ENABLED=false and pages above the direct limit → ValidationException
• sharding enabled and pages at or below OVERSIZED_PDF_SOFT_LIMIT → admitted
• pages above the soft limit → contact-support rejection
• shard/profile failures expose user_message + reason, and keep internal_message off to_client()

This does not implement #121 productionization (shard pipeline / 300MB upload). It only locks the current rejection policy.

Verification

• make lint — All checks passed
• make typecheck — 0 errors, 0 warnings, 0 informations
• cd apps/worker && uv run pytest -p no:postgresql tests/unit/test_oversized_pdf_policy.py -q — 7 passed

-p no:postgresql is only to skip the autoloaded pytest-postgresql plugin in this environment (no libpq). The tests do not use Postgres.

Deployment Notes

• No new or changed environment variables
• No database migrations, queue changes, storage changes, or release-order requirements
• Backwards compatible; rollback is reverting this PR

Checklist

• Tests were added or updated when behavior changed
• Public docs, examples, or OpenAPI contracts were updated when needed — N/A (test-only)
• Database migrations are idempotent and safe to deploy — N/A
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change — none