bufio.Scanner has a fixed max-token size (SSEScanBufMax, 1MB). Codex's
native image_generation streams a full base64 image in a SINGLE SSE
data line (response.image_generation_call.partial_image and the final
image_generation_call item), which regularly exceeds 1MB for anything
larger than a thumbnail. Scanner.Scan() then fails with:
stream read error: bufio.Scanner: token too long
killing the whole turn after the model already generated the image.
Replace bufio.Scanner with bufio.Reader.ReadString, which grows to fit
a line of any length instead of erroring past a fixed cap. SSEScanBufMax
is now unused and removed; SSEScanBufInit is reused as the reader's
initial buffer size.
