Add IMessageBatch / IMessageOutbox plumbing for projection-emitted me…

[jeremydmiller]

…ssages

Closes #84.

Marten projections can publish side-effect messages on commit (transactionally with the projection write) via Wolverine.Marten's MartenToWolverineMessageBatch. Polecat's projection daemon had no equivalent — the IProjectionBatch.PublishMessageAsync hook was a Task.CompletedTask stub.

This adds the Polecat-side plumbing only — a future Wolverine.Polecat will implement IMessageOutbox to bridge into Wolverine's outgoing- message machinery. The user explicitly scoped this PR Polecat-only.

Added

• Polecat.Events.Aggregation.IMessageBatch — extends JasperFx.Events.IMessageSink with BeforeCommitAsync(token) and AfterCommitAsync(token) hooks. The batch buffers messages in PublishAsync; the implementer chooses which hook to flush in based on the desired delivery guarantee.

• Polecat.Events.Aggregation.IMessageOutbox — factory that vends a fresh IMessageBatch per projection daemon batch. The signature takes the public IDocumentSession (rather than internal DocumentSessionBase) so downstream integrations don't need InternalsVisibleTo; IDocumentSession already exposes ITransactionParticipantRegistrar for enrollment.

• Polecat.Events.Aggregation.NulloMessageOutbox — singleton no-op default (drops messages, fires no hooks). Apps that don't integrate a message bus pay zero overhead.

• EventStoreOptions.MessageOutbox property defaulting to NulloMessageOutbox. This is the registration point for downstream integrations.

Wired through PolecatProjectionBatch

• New _messageBatch field + _messageBatchGate semaphore for thread-safe lazy initialization. Stays null when no projection in this batch publishes.

• PublishMessageAsync(object, string) — was a Task.CompletedTask stub; now lazily creates the batch via IMessageOutbox.CreateBatch and forwards the message via cached generic IMessageSink.PublishAsync<T> MethodInfo. The reflection lookup is done once at static init; per-publish cost is one MakeGenericMethod + one Invoke (off the per-event hot path — only fires when a projection explicitly emits a side-effect).

• New PublishMessageAsync(object, MessageMetadata) overload mirroring the same pattern; satisfies the JFx.Events.IProjectionBatch contract added in the canonical interface (default impl forwards tenant-only; we wire the metadata-aware path so downstream integrations can stamp correlation / causation / headers).

• ExecuteAsync passes the snapshot batch into the resilience lambda's state tuple. BeforeCommitAsync fires inside the SQL transaction (right before tx.CommitAsync) so an at-least-once outbox can persist its envelopes in the same transaction. AfterCommitAsync fires outside the resilience pipeline (after success) so it does not re-fire on a transient SQL retry.

Tests

src/Polecat.Tests/Daemon/projection_message_outbox_tests.cs — 4 cases:

• No publish in the batch → IMessageOutbox.CreateBatch is never called and no hooks fire (the common no-op path)
• Publish creates the batch exactly once and forwards every published message with the right type and tenant id
• BeforeCommit + AfterCommit hooks both fire after ExecuteAsync completes, in the right order (publish → before → after)
• The metadata overload correctly extracts tenant id (default IMessageSink impl) and forwards through the same path

All 4 pass against SQL Server 2025 docker on net9.0.