fix: include page citation assets in result zip

[suguanYang]

Summary

• Include referenced page_citation_assets/... files in worker-generated result.zip bundles.
• Preserve result-relative page citation asset paths so SDK parsed-storage sync can copy them into downstream storage.
• Add a worker parse contract test that drives the task boundary and verifies the uploaded result ZIP contains referenced page citation assets while excluding unreferenced files.

Verification

• uv run pytest apps/worker/tests/contract/test_parse_task_contract.py::test_parse_task_result_zip_includes_page_citation_assets -q
• uv run pytest apps/worker/tests/contract/test_parse_task_contract.py apps/worker/tests/contract/test_page_memory_retrieval_contract.py apps/worker/tests/contract/test_page_memory_node_assembler_contract.py -q
• uv run ruff check packages/shared-python/shared/services/storage/zip_result_resources.py packages/shared-python/shared/services/storage/zip_package_writer.py packages/shared-python/shared/services/storage/zip_result_service.py apps/worker/tests/contract/test_parse_task_contract.py
• uv run pyright packages/shared-python/shared/services/storage/zip_result_resources.py packages/shared-python/shared/services/storage/zip_package_writer.py packages/shared-python/shared/services/storage/zip_result_service.py
• Manual local ZIP probe confirmed page_citation_assets/page-6.png appears in generated ZIP members.

Deployment Notes

• No new environment variables.
• No database migration.
• Storage behavior changes for newly generated result ZIPs only: referenced page citation PNGs are now included inside the ZIP in addition to existing raw artifact upload.
• Existing completed jobs are not rewritten; reparsing is required for old result ZIPs to include page citation assets.

Checklist

• Tests were added or updated when behavior changed
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change