fix: remove trailing separators from CSV export

[Kumar-s75]

Summary

• format CSV list values with join(', ') instead of trimming a manually appended separator
• remove trailing commas from tag and assigned-user fields
• preserve CSV escaping for embedded double quotes
• add tests for multiple values, quoted values, and empty lists

Verification

• npx jest src/services/BoardApi.spec.js --runInBand --config='{"rootDir":".","moduleFileExtensions":["js"],"transform":{"^.+\\.js$":"babel-jest"}}'
• npx eslint src/services/BoardApi.js src/services/BoardApi.spec.js
• npm run build

All checks passed. The build reports existing Sass deprecation and bundle-size warnings.

Closes #8361

[github-actions]

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[grnd-alt]

looks good to me, but please don't put the test file just in the services folder put it either in src/services/tests or in src/tests

[Kumar-s75]

Thank you for the review, @grnd-alt. I have moved the test file to src/services/tests/BoardApi.spec.js and updated its import as requested.

The change has been pushed in commit f014d497e. All three focused Jest tests, ESLint, and whitespace checks pass.

The update is ready for another review.