buildAssistPrompt(action, selection) returns the {system, messages}
payload for rewrite/shorten/expand. Enforces 8KB selection cap and
typed errors with codes (unknown_action, bad_selection,
empty_selection, selection_too_large) so the IPC layer can map them
to user-friendly messages.
applyAssistResult(original, newText) trims the response and rejects
empty/unchanged payloads so a no-op streaming chunk doesn't churn
the editor.
15 new tests cover action coverage, prompt content, all four error
codes, cap-edge (exactly 8KB OK, 8KB+1 rejected), and trim semantics.
Amit Haridas
