Follow-up: JsonTypeInfo<T> source-gen (AOT) for MTConnect JSON hot-path types

[ottobolyos]

Origin

Ultrareview dime cycle 1 on #249 (fix/json-serializer-options-per-call-leak). Recorded as TRACKED per CONVENTIONS §1.0d-trigies-septies — belongs in its own campaign PR (touches every serialised type in both JSON assemblies), split out of #249's atomic scope.

Problem

#249 freezes the JsonSerializerOptions singletons so their reflection metadata cache is built once. That eliminates the per-call LCG-DynamicMethod leak observed on DIME production hosts. But the underlying serializer still uses runtime reflection to introspect every reachable type in the MTConnect graph on first serialization of that type — 10 Streams.Json classes in MTConnect.NET-JSON, 25 in MTConnect.NET-JSON-cppagent, plus the Devices/Header/Assets/Errors trees.

That reflection cost:

• Pays a large first-request latency the first time each response type is serialised.
• Blocks AOT scenarios (ReadyToRun, NativeAOT, iOS/tvOS mono, .NET 10 AOT publish) — the trimmer + AOT compiler cannot see through the runtime reflection, so trimmed AOT builds silently drop the metadata and fail to serialise at runtime.
• Emits IL2026 / IL3050 trimming warnings under PublishTrimmed=true, currently muted because the whole assembly isn't AOT-audited.

Proposal

Introduce System.Text.Json source generation across both assemblies:

• Add a [JsonSerializable(typeof(...))]-marked JsonSerializerContext per assembly: MTConnectJsonContext in MTConnect.NET-JSON, MTConnectCppagentJsonContext in MTConnect.NET-JSON-cppagent.
• Enumerate every serialised type on the hot path (grep every JsonSerializer.Serialize / Deserialize call, walk from the root Response/Streams/Devices/Assets/Errors types, close the transitive closure).
• Wire the generated context into JsonFunctions.DefaultOptions / IndentOptions via TypeInfoResolver on both singletons.
• Set TrimmerRootAssembly / PublishTrimmed=true on a representative agent host and confirm the trimmer + AOT publish walk succeeds with zero IL2026 / IL3050 warnings on the JSON path.
• RED-first: pin every root Response type has a JsonTypeInfo<T> on the context; pin trimmed publish output produces byte-identical JSON to the reflection path.

Non-goals

• Not in scope: source-gen for the XML surrogate assemblies (separate campaign, different serializer stack).
• Not in scope: switching the default resolver at runtime — the source-gen context should be additive (chained onto DefaultJsonTypeInfoResolver), not replacing.

Acceptance

• Both assemblies expose a JsonSerializerContext covering every serialised type in the reachable graph.
• Reflection-path baseline stays byte-identical (regression pins).
• PublishTrimmed=true on a representative host publishes cleanly with zero IL2026 / IL3050 warnings from the JSON assemblies.
• Full unit + E2E suites green on bluefin.