#629 Add tests for export service file formats

[vic-Gray]

Write tests verifying CSV, Excel, and PDF exports produce valid output.\n\nWhy: Export bugs are often discovered by users rather than tests.\n\nHints:\n- backend/src/export/export.service.ts\n- Verify column headers and row counts

[daveedAJ]

@daveedAJ has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello team,

I’d love to work on this issue because reliable export functionality is important for reporting and user trust. I have experience writing backend tests for generated files and data validation, and I’m confident I can add solid coverage for CSV, Excel, and PDF export outputs.

Thank you for considering me.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @daveedAJ to this issue.

[Hakimziyech18]

@Hakimziyech18 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi Goodday, I'd love to be assigned this issue to resolve and earn some points, Thank you.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Hakimziyech18 to this issue.