Error results reach the wire as text plus `isError: true`, but the
text-only envelope omitted the flag. An error up to 15 bytes above the
result budget passed both the delivery and transport checks and was sent
as the original diagnostic instead of taking the oversized-error recovery
path.
Rename the error representation to "error-text" and size its envelope
with the flag, so ToolResultDelivery and the error transport share the
complete delivered shape. Successful structured results are unchanged.
Fixes#2557
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>