The old assertions only searched REACTIVE_RUNTIME for substrings; they never
touched buildApiRequest and were not anchored to the content-type line they
claimed to guard, so they could not detect drift on either side. Replace
with a fixture-driven test that runs both implementations on the same
(path, method, input) cases and compares the actual request they produce.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>