M2: Unknown* concrete-type serialization does not reproduce the raw document (F07) #19

Source: multi-agent review of PR #16, verifier CONFIRMED · medium · public API · all seven Unknown* carriers. 2026-08-13. M-anchor: M2 opening hardening batch.

UnknownOpenCodeError.Payload is public and not ignored (src/OpenCode.Sdk/Models/UnknownOpenCodeError.cs:32). The union converter is attached to the OpenCodeError base and writes Payload.WriteTo only when dispatch goes through the base; concrete source-generated metadata treats the carrier as an ordinary record and emits {"_tag":...,"Payload":{...}} instead of the preserved original JSON. Same shape on the other six carriers. SDK's own HTTP I/O is unaffected (uses union-base metadata); consumer serialization of a concrete carrier is affected.

Fix (verifier-corrected): change UnionEmitter to attach a concrete-type converter whose write path calls Payload.WriteTo. Do not use [JsonIgnore] alone — that would keep only the marker and discard the preserved body. Keep Payload public. Add concrete-type round-trip tests across representative carriers. Regenerate committed output.