JasperFx.Events 1.29.0: per-message metadata on side-effect PublishMessage

[jeremydmiller]

Summary

Adds an independent path through the side-effect publishing pipeline that lets projection authors attach per-message metadata (correlation id, causation id, tenant, headers, user name) when calling PublishMessage from inside RaiseSideEffects.

Motivation

Wolverine's Marten integration builds a fresh MessageContext for side-effect publishing, which means the outgoing Wolverine envelope is never stamped with the originating event's correlation id. Users have no per-message override because PublishMessage only accepts the message object itself. See JasperFx/wolverine#2545 for the motivating scenario.

This PR opens the path. The Marten and Wolverine PRs will follow once this is released.

New surface

• MessageMetadata struct implementing IMetadataContext — a lightweight value carrier. Headers lazy-allocates so unused metadata stays allocation-free.
• IMessageSink.PublishAsync<T>(T message, MessageMetadata metadata) — new overload with a default implementation forwarding to the tenant-only overload.
• IEventSlice<T>.PublishMessage(object message, MessageMetadata metadata) + PublishedMessagesWithMetadata() — same story.
• IProjectionBatch.PublishMessageAsync(object message, MessageMetadata metadata) — same story.

Internal routing

EventSlice<TDoc,TId> keeps the original PublishedMessages list untouched and adds a separate PublishedMessagesWithMetadata list. The three projection dispatch sites (AggregationRunner, inline single-stream, and inline multi-stream projections) iterate both lists independently, so the original path remains byte-identical for callers that don't opt in.

Compatibility

All new interface members carry default implementations that drop the metadata and forward to the existing signatures. Every existing implementation of IMessageSink / IEventSlice<T> / IProjectionBatch compiles and runs unchanged. Strictly additive.

Test plan

• dotnet test src/EventTests — 219/219 pass
• dotnet test src/EventStoreTests — 72/72 pass
• CI across net8 / net9 / net10

Downstream

Once this ships as JasperFx.Events 1.29.0:

1. Marten: add an overload on ProjectionBatch.PublishMessageAsync forwarding to IMessageSink.PublishAsync(message, metadata).
2. Wolverine: implement the metadata-aware overload on MartenToWolverineMessageBatch, mapping to DeliveryOptions (Automatic correlation ID propagation through event-sourced handler chains wolverine#2545).

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