The streaming reader's NUL check only walked completed lines collected
from streamLinesFromFile, so a binary blob whose first newline lay past
the byte budget (videos, archives, packed JSON) left collectedLines
empty and slipped through to the firstLineExceedsLimit branch — which
emitted the decoded preview as text instead of the intended refusal.
Sniff firstLinePreview alongside collectedLines so the existing refusal
fires uniformly. Also added a regression test that uses a 256 KiB blob
with no 0x0A bytes to actually exercise the firstLineExceedsLimit
path — the previous 6-byte test fit in one collected line and never
covered the bug.
Fixes#3448
roboomp
93f1019ead