Builtin `jq -Rs` on empty stdin produces no output instead of `""`

[chaliy]

Summary

When stdin is empty, jq -Rs '.' should produce "" (an empty JSON string). Bashkit's builtin jq produces no output at all, causing downstream jq pipelines to fail or produce malformed JSON.

Reproduction

# FAIL — empty stdin with -Rs
printf '' | jq -Rs '.'
# expected: ""
# actual:   (no output)

# FAIL — breaks response reconstruction
text_content=""
printf '%s\n%s' "$(printf '%s' "$text_content" | jq -Rs '.')" "[]" | jq -s '.'
# expected: ["", []]
# actual:   jq error or malformed output (first element missing)

Spec tests to add

### jq_raw_slurp_empty_stdin
# jq -Rs on empty stdin should produce empty JSON string
printf '' | jq -Rs '.'
### expect
""
### end

### jq_raw_slurp_newline_only
# jq -Rs on newline-only stdin
printf '\n' | jq -Rs '.'
### expect
"\n"
### end

### jq_raw_slurp_normal
# jq -Rs on normal input (no regression)
printf 'hello' | jq -Rs '.'
### expect
"hello"
### end

Where to fix

crates/bashkit/src/builtins/jq.rs — when -R (raw input) and -s (slurp) are combined, the jq builtin should read all of stdin as a single raw string. When stdin is empty, this should produce an empty string "", not skip output entirely. The issue is likely an early return when stdin is empty/EOF.

Real-world impact

High — The wedow/harness OpenAI and Anthropic providers reconstruct the full API response after streaming by combining text content and tool calls via jq -s. When the LLM response is tool-call-only (no text), text_content is empty, and jq -Rs produces nothing, causing the tool calls array to shift into the wrong position in the reconstructed response JSON.