fix(jq): ignore unread invalid stdin under -n

[chaliy]

Requested by Михайло · project thread

What changed

With -n, jq no longer reports invalid JSON on stdin that the filter never reads. A stdin parse error is reported only after input or inputs reads past the last valid value, which matches jq 1.7.

Why

jq -n is the usual way to build JSON from --arg values. Scripts often run it while stdin holds something else, such as a pipe or a while read loop's here-doc. bashkit printed the right output, then added jq: invalid JSON ... and exited 5. That breaks set -e scripts that work under real bash. Found during the BashBox re-run (#2657).

Before/After (verified)

printf 'not json' | jq -n '"x"'; echo "rc=$?"
while read -r l; do jq -n --arg v "$l" '$v'; done <<X
one
X

• Real bash and jq 1.7: "x", then rc=0, then "one".
• Before: each output was followed by jq: invalid JSON: expected ident ..., and the first command ended with rc=5.
• After: output is identical to real bash.

The following cases behave the same as jq 1.7:

• printf '1 x' | jq -n input prints 1 with status 0.
• printf '1 x' | jq -n '[inputs]' still fails with status 5.

Risk

Low. The change only applies with -n, and only to whether a parse error is reported. Values are still parsed eagerly as before.

Checklist

• failing tests written first (null_input_ignores_unread_invalid_stdin, null_input_reports_invalid_stdin_once_reached)
• jq unit tests (348), jq spec tests, clippy -D warnings, fmt
• knowledge/log.md updated

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