Add store-agnostic AllDatabases() to IEventStore (#387)

[jeremydmiller]

Closes #387.

What

Adds a store-neutral database accessor to JasperFx.Events.IEventStore:

ValueTask<IReadOnlyList<IEventDatabase>> AllDatabases();

Shipped as a default interface member that returns an empty array as a stand-in, so every existing IEventStore implementer keeps compiling and degrades gracefully until it overrides. Signature mirrors Marten's existing IMartenStorage.AllDatabases().

Why

Store-agnostic monitoring/tooling (e.g. CritterWatch) needs an IEventDatabase to call the read abstractions — AllProjectionProgress(...) plus FetchDeadLetterCountsAsync(...) / CountDeadLetterEventsAsync(...) from #356 — but there was no store-neutral way to obtain one:

• A Marten + Wolverine host registers IEventStore in DI (concrete Marten.DocumentStore) but does not register IEventDatabase, so GetServices<IEventDatabase>() is empty.
• The registered IEventStore could only yield its databases through Marten's own IMartenStorage.AllDatabases() — concrete-store types a store-agnostic consumer must not reference.
• IEventStore itself exposed only DatabaseCardinality — no path to IEventDatabase.

AllDatabases() closes the loop: consumers resolve the already-registered IEventStore and enumerate IEventDatabase store-agnostically.

Follow-ups (real implementations)

The default returns empty; the concrete stores will override:

• Marten — Override IEventStore.AllDatabases() on DocumentStore (jasperfx#387) marten#4570 (delegate to IMartenStorage.AllDatabases())
• Polecat — Override IEventStore.AllDatabases() on DocumentStore (jasperfx#387) polecat#156 (project ITenancy.AllDatabases())

Tests

AllDatabasesDefaultTests exercises the default member against a bare IEventStore and asserts an empty result, mirroring DeadLetterCountDefaultsTests from #356.

Companion to #356; unblocks CritterWatch#213 (ProjectionDeadLetters alert).

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