Preserve duplicate-key refusal at end of input #496

🤖 Generated by the Agentic Engineer

Problem

The installed raw JSON stream library can accept repeated decoded fields when a final scalar is followed by EOF without a newline. A trailing streaming result replaces the earlier refusal. Existing observer callers append newlines, so this is a direct-library completeness defect.

Proposed direction

Repair the owning observation or output boundary, with a regression that fails on current main before changing the implementation. Preserve the existing conservative behavior and public interface.

Acceptance criteria

• Produce one accumulated verdict for the complete raw input and retain a duplicate refusal through EOF.
• Preserve valid JSON with and without a final newline and supported complete page streams.
• Exercise nested/scalar duplicates and normal observer callers without claiming a bypass they do not exhibit.

Size: S/M. Part of #223.