Address original data download review comments

[theosanderson-agent]

Summary

Follow-up for #5856 addressing several review comments on the original-data download feature.

Changes:

• preserve original submission IDs in the downloaded metadata id column and single-segment FASTA headers
• preserve original multi-segment FASTA IDs instead of rewriting them with accession-version prefixes
• move group authorization out of countOriginalData / streamOriginalData
• remove the unused accession-version filter plumbing from original metadata helpers
• clear MDC in a finally block for the zip streaming response
• parse structured backend error responses in the client helper
• remove inconsistent client-side post-fetch size validation and add an accessible dismiss label
• remove redundant required = true schema annotation

I left the writer close/flush comments alone: these writers are wrapping an active ZipOutputStream entry, so closing the wrapped writer would close the whole zip stream before later entries are written.

Checks

• JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64 ./gradlew test --tests org.loculus.backend.controller.submission.GetOriginalDataEndpointTest
• JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64 ./gradlew ktlintCheck
• npm run check-types (passes with existing EditableSequences.spec.ts deprecation hints)
• npx prettier --check src/components/SearchPage/DownloadDialog/DownloadOriginalDataButton.tsx src/services/backendClientSideApi.ts
• git diff --check

Note: repo-wide npm run check-format currently fails on existing unrelated lint issues in EditableSequences.spec.ts, SearchForm.spec.tsx, and FileUploadComponent.tsx.

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