Refactor generateTSVString to remove mutation and improve functional style

[SarthakJagota]

The generateTSVString function currently uses mutation (push) and imperative accumulation inside a forEach loop. There is also a TODO comment suggesting refactoring the logic to use map/reduce to avoid mutating tsvRows.

This issue proposes refactoring the function to:

Replace forEach + push with flatMap/map

Remove mutation-based accumulation

Eliminate the existing @ts-expect-error

Preserve the exact output format and behavior

This will improve readability, maintainability, and TypeScript safety without changing functionality.