Playwright backend: 'Converting circular structure to JSON' on React inputs (Medplum Create click) #1908

The GUI save on Medplum's web app fails at the Create click with "Page.evaluate: Converting circular structure to JSON (HTMLInputElement __reactFiber...)": an evaluate call returns or serializes a DOM element with React internals. Return only plain data from page evaluate calls. Data: results/frozen-969cb939/medplum_patient.

Found by the API-step generalization experiment (internal#106); data in #1906 under results/. Fix generically (no app-specific code), fail closed, and add a real-app regression test.