fix: upload local MinerU shards when HeadObject returns 403

[suguanYang]

Summary

• Fresh tmp/mineru-shards/... keys are not in S3 yet. Staging HeadObject returned 403 (not 404), which aborted URL mode and forced a slow direct upload to MinerU.
• Treat 403/Forbidden on the existence probe as “not verified”, then upload the local shard and continue S3 URL mode. Other verify errors still fall back to direct upload.
• Reuse of existing uploads/ objects is unchanged.

Test plan

• uv run pytest apps/worker/tests/unit/test_mineru_pdf_service.py
• After deploy, re-run Exercise_Prescription_in_Cardiac_Rehabilitation.pdf on staging and confirm Logfire shows url_mode_source_unverified then url_mode_source_uploaded / Using S3 URL mode, not an immediate direct-upload fallback from verify_source_object

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