CI clippy ran with --all-features but without --all-targets, which
enabled the unused 'test-helpers' feature (making build_api_payload
present) while not compiling the #[cfg(test)] block that used it.
Result: dead_code warning → -D warnings failure.

Remove the unused 'test-helpers' feature entirely and gate the helper
purely on cfg(test). The feature had no external consumers.

Verified locally with: cargo clippy --workspace --all-features -- -D warnings