Fix ancillary store outbox schema for projection side effects (GH-2887)

[jeremydmiller]

Problem

Fixes #2887.

In a modular monolith with a main Marten store plus an ancillary store on a separate physical database, integrated via IntegrateWithWolverine(x => x.SchemaName = "debtors"), an async multi-stream projection that calls slice.PublishMessage(...) in RaiseSideEffects fails with:

42P01: relation "public.wolverine_incoming_envelopes" does not exist

The projection-batch outbox writes its envelopes to public.wolverine_* (the main store's schema) instead of the ancillary store's configured debtors schema. On the ancillary database those public tables don't exist, so the batch fails and the side-effect message is never delivered.

A shared database masks the bug — the main store's public.wolverine_* tables happen to exist there, so the wrong-schema write silently succeeds. It only surfaces when the ancillary store is on its own database.

Root cause

MartenToWolverineOutbox.CreateBatch(session) builds a fresh MessageContext, whose Storage defaults to runtime.Storage — the main/default message store (schema public). MartenEnvelopeTransaction then builds schema-qualified envelope SQL from that store and runs it on the projection's (ancillary) session connection.

The same outbox class is used for every store and had no idea which store it belonged to. By contrast, OutboxedSessionFactory<T> correctly calls OverrideStorage(...) to retarget the ancillary store — the projection-batch path never did.

Fix

Thread the ancillary store's marker type into the outbox and retarget storage when present:

• MartenToWolverineOutbox takes an optional Type? storeType; when set, calls context.OverrideStorage(runtime.Stores.FindAncillaryStore(storeType)).
• MartenOverrides exposes protected virtual Type? StoreType => null (main store keeps default storage, unchanged); MartenOverrides<T> overrides it to typeof(T).

Test

Adds Bug_2887_ancillary_outbox_schema_on_separate_database — an ancillary store on a separate physical database (bug2887_refs) with a per-store envelope schema (debtors), durable local queues, and an async multi-stream projection that publishes a side effect. Fails with the exact 42P01 before the fix; passes after.

Verification

• Bug_2887 repro: fails before (exact 42P01), passes after.
• Regression run across related ancillary/outbox tests (multi_stream_projection_with_side_effects_on_ancillary_store, end_to_end_publish_messages_through_marten_to_wolverine, Bug_2669, Bug_2382 ancillary): 11/11 passed.

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