Fix parallel PDF API script env handling

[compair-steven]

Summary

Fixes scripts/user/process-pdf-parallel-through-api.sh so it can be run with documented environment overrides and on the default Bash shipped with macOS.

Changes:

• Documents UNST_API_KEY as required.
• Adds UNST_API_ENDPOINT support instead of hard-coding the public endpoint.
• Gives STRATEGY, BATCH_SIZE, PDF_SPLIT_PAGE_SIZE, and PDF_SPLITS_DIR defaults before use.
• Avoids unbound-variable failures when UNST_API_KEY is missing.
• Replaces mapfile usage with a Bash 3.2-compatible loop.
• Supports both md5sum and BSD/macOS md5 -q in the parallel script and split helper.

Validation

I tested the script against the hosted API with a valid UNST_API_KEY using a one-page split of example-docs/pdf/layout-parser-paper-fast.pdf:

PDF_SPLITS_DIR="$tmp" \
BATCH_SIZE=1 \
STRATEGY=fast \
scripts/user/process-pdf-parallel-through-api.sh "$pdf"

The script produced a valid split JSON file, combined it successfully, and the combined output contained 25 JSON elements:

jq length "$tmp/${base}-output-${md5}_split-10_strat-fast/${base}_combined.json"
# 25

I also checked the updated shell scripts with bash -n.

[cubic-dev-ai]

2 issues found across 2 files

<sub>Shadow auto-approve: would not auto-approve because issues were found.

Re-trigger cubic</sub>

[cubic-dev-ai]

0 issues found across 2 files (changes from recent commits).

<sub>Shadow auto-approve: would auto-approve. Improves shell script portability, env var handling, and validation. Low risk, no change to core logic or API endpoints.

Re-trigger cubic</sub>

[compair-steven]

Thanks for the quick review! I pushed a follow-up that validates BATCH_SIZE as a positive integer before the batching loop, tightens PDF_SPLIT_PAGE_SIZE to the same positive-integer shape, and checks that checksum generation produced a non-empty value before constructing split/output paths.

I also verified the failure paths for BATCH_SIZE=0, non-numeric BATCH_SIZE, PDF_SPLIT_PAGE_SIZE=0, and a missing PDF input. Hopefully this addresses the concerns :)