[jeremydmiller]

Final follow-up in the raw-JSON series (#3407, #3442): the JsonSerializerOptions argument on ReceiveRawJson() and PublishRawJson() has never had any effect on any code path. This makes it real.

Why it was dead

• Outgoing: Envelope.Data lazily serializes the message through the endpoint's serializer before the mapper runs, so JsonOnlyMapper's Message-fallback branch — the only consumer of the stored options — was unreachable. Deleted, along with the now-parameterless options plumbing in the mapper.
• Incoming: the mapper never looked at the options at all, and raw records carry no content-type, so HandlerPipeline.serializerFor fell through to the global default serializer.
• Bonus: the generic ReceiveRawJson<T>(options) overload dropped the argument entirely when delegating to the Type overload.

The fix

Supplying options now sets the endpoint's DefaultSerializer to a SystemTextJsonSerializer wrapping them — one mechanism that covers both directions coherently ("this endpoint speaks JSON in this dialect"):

• Outgoing bodies serialize through Envelope.Data → endpoint serializer → the supplied options.
• Incoming: JsonOnlyMapper now stamps envelope.Serializer (and ContentType) from the endpoint, mirroring what the base EnvelopeMapper does — without this, the null content-type on raw records routes deserialization to the global serializer regardless of endpoint configuration. Stamping ContentType also keeps the endpoint's serializer resolvable on a durable-inbox recovery replay, where the transient Serializer reference is lost.

Compatibility: gated on options != null. Bare ReceiveRawJson<T>() / PublishRawJson() callers see zero change — Wolverine's defaults (camel-cased, case-insensitive reads) still apply, and body bytes are byte-identical to before. Only callers who explicitly passed options see a change, and it's the behavior the parameter has always advertised.

Tests

• Snake-cased external JSON ({"color_name": ...}) round-trips through ReceiveRawJson<T>(SnakeCaseLower options) — impossible with the default case-insensitive camelCase serializer, so it proves the options drive deserialization.
• A plain Confluent consumer reads a PublishRawJson(new JsonSerializerOptions()) record and asserts a Pascal-cased body — distinct from Wolverine's camel-cased default, proving the options drive serialization.

(Test-infra note: both tests use per-run unique topics and AutoOffsetReset.Earliest; the librdkafka Latest default silently discards records produced while a consumer group is still joining a freshly provisioned topic.)

Docs

The Kafka interop upgrade warning gains a bullet stating the options now work and what they control.

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