fix(fetchers): surface malformed body errors

[chaliy]

What

Treat malformed zero-byte response bodies as fetch errors instead of successful empty responses.

Closes #98.

Why

read_body_with_timeout() collapsed a body-stream failure before the first chunk into (empty, false). That let malformed chunked responses look like successful 200 OK fetches with empty content, and the same path could also create misleading zero-byte files under save_to_file.

How

• change read_body_with_timeout() to return FetchError when the body stream fails before any bytes are read
• preserve partial-body behavior for mid-stream failures by returning the partial bytes as truncated=true
• propagate the new result through the default fetcher and direct llms.txt docs-site path
• add regressions for malformed chunked responses in both normal fetch and save_to_file flows

Risk

• Low
• Changes only the malformed-body error path; successful responses, timeouts, size caps, and partial-body truncation stay on the existing behavior

Checklist

• Unit tests are passed
• Smoke tests are passed
• Documentation is updated
• Specs are up to date and not in conflict
• cargo fmt --all is passed
• cargo clippy --workspace --all-targets -- -D warnings is passed
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps is passed
• cargo build --workspace --exclude fetchkit-python --release is passed