fix: fail incomplete pr content fetches

[Desel72]

Summary

Keep scoring_data_stored false when PR file content fetching cannot complete.

• Throw when a PR file fetch has no headSha, instead of returning before content fetch.
• Throw when GraphQL content fetching still fails at the minimum batch size, instead of skipping that batch and continuing.
• Let the existing BullMQ retry/failure path handle incomplete fetches rather than marking the PR generation complete.

Related Issues

Closes #37

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Testing

• npm run build
• npm run lint
• npm run format:check
• git diff --check

Checklist

• I have read the Contributing Guide
• Code builds without errors
• New and existing tests pass (if applicable)
• Documentation updated (if applicable)
• No unnecessary dependencies added