[DR-74493] fix pdf validation config in prod

[anniebtran]

Summary

This feature is controlled by a backend setting that is enabled in staging but not prod (yet): Settings.decision_review.pdf_validation.enabled

This fixes the configuration used in the proxy service that was previously built but never fully switched on (see more details here) — we are now ready to use it, so I've been trying to fix the configurations to make sure it all works as expected.

I recently learned that we are able to use the Decision Reviews API key in dev and staging as originally designed (through this service's configuration inheriting the DR configuration) — but it didn't work in production when I tried making some test calls using the DR prod API key. The test calls did work when I used prod's Benefits Intake API key, so I've added a conditional to use the proper API key in production

Related issue(s)

department-of-veterans-affairs/va.gov-team#74493

Testing done

• Tested that dev still works as it previously did, will also check in staging
• Checked that Settings.benefits_intake_service.api_key || Settings.form526_backup.api_key returns the proper API key in production. Will check in production that API call using service works before enabling the feature

What areas of the site does it impact?

Appeals Backend (on a feature that is not enabled in production yet)

Acceptance criteria

• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs