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