AOT: route JsonUnicodeBodyConverter (SqlServiceBroker) through GetTypeInfo #515

Goal

Give Chatter.MessageBrokers.SqlServiceBroker's JsonUnicodeBodyConverter a metadata-aware serialization path, following the pattern #509 establishes for the Chatter.MessageBrokers Body Converters.

Current state on master (verified at 7408c31)

src/Chatter.MessageBrokers.SqlServiceBroker/src/Chatter.MessageBrokers.SqlServiceBroker/JsonUnicodeBodyConverter.cs is 25 lines. Its serialization calls are:

• line 10 — Convert<TBody>(byte[] body) returns ChatterJson.Deserialize<TBody>(Stringify(body)).
• line 20 — Stringify(object body) returns ChatterJson.Serialize(body).

It has no constructor, takes no options, and has a parameterless default. It handles a UTF-16 payload (ContentType is "application/json; charset=utf-16"), with Encoding.Unicode for the byte conversions.

Corrections to earlier notes

• The quoted code does not exist. This issue previously stated that the converter "still calls JsonSerializer.Deserialize<TBody>(Stringify(body), ChatterJson.Options) / Serialize(body, ChatterJson.Options) directly". It does not, and it could not: ChatterJson.Options is internal (src/Chatter.MessageBrokers/src/Chatter.MessageBrokers/ChatterJson.cs:24), and src/Chatter.MessageBrokers/src/Chatter.MessageBrokers/Properties/AssemblyInfo.cs grants production InternalsVisibleTo only to Chatter.MessageBrokers.AzureServiceBus and Chatter.MessageBrokers.Reliability.Cosmos — not to this package. The converter consumes the published Wire Serialization capability, which is exactly what src/Chatter.MessageBrokers/CONTEXT.md prescribes for a sibling package.
• "Hardcoded to the reflection-based default" is therefore the wrong framing. The converter is correctly coupled to the published capability; the AOT problem is that the capability itself has no metadata-aware form yet. That is the real gap, and it is shared with Chatter.MessageBrokers.RabbitMQ's RabbitMqBodyConverter, which has the identical shape.
• AOT: route Body Converter JSON through JsonTypeInfo instead of the annotated JsonSerializer overloads #509 has not landed here. No file in the repository routes serialization through options.GetTypeInfo. There is no recipe to apply yet — this issue is a dependent of AOT: route Body Converter JSON through JsonTypeInfo instead of the annotated JsonSerializer overloads #509, not a follow-on to a completed change.
• The RuntimeFeature.IsDynamicCodeSupported guard pattern attributed to AOT: ChatterJson's shared options field initializer eagerly builds the reflection-based resolver #431 and AOT: route Body Converter JSON through JsonTypeInfo instead of the annotated JsonSerializer overloads #509 does not exist anywhere in the repository.

Design constraint

Adding a public JsonSerializerOptions constructor parameter would make the serializer configuration a caller-facing surface, which src/Chatter.MessageBrokers/CONTEXT.md's Wire Serialization term explicitly warns against, and it would not even give this package access to Chatter's own options across the package boundary. The likely correct shape is to extend the published capability with a metadata-aware entry point that JsonUnicodeBodyConverter and RabbitMqBodyConverter both call, rather than to publish options on each converter. Resolve that question in #509 and apply the answer here.

Scope

• src/Chatter.MessageBrokers.SqlServiceBroker/src/Chatter.MessageBrokers.SqlServiceBroker/JsonUnicodeBodyConverter.cs, lines 10 and 20.
• Adopt whatever metadata-aware seam AOT: route Body Converter JSON through JsonTypeInfo instead of the annotated JsonSerializer overloads #509 settles on. Do not introduce a package-local variant.
• Any new public constructor parameter is a public-API-surface and SemVer question. Flag it; do not characterize it here.
• Preserve the UTF-16 encoding behavior and byte-for-byte wire output exactly.
• Add coverage: Chatter.MessageBrokers.SqlServiceBroker is referenced by no AOT smoke test, because no AOT smoke-test project exists yet. AOT Phase 0: Infrastructure and red baseline #276 creates the harness and AOT: cover AzureServiceBus, SqlServiceBroker and SqlChangeFeed receiver registration in the Native AOT smoke tests #429 tracks adding this module to it.
• Measure real publish-time and compile-time diagnostic counts before and after, from a clean tree.

Depends on #509 and, transitively, on #279 and #276.

Part of #275.