GH-5002 The leader's heartbeat reads shard names and databases, not the full event store usage

[jeremydmiller]

Closes #5002. Reported downstream by @uniquelau in JasperFx/marten#5652.

EventStoreAgents.SupportedAgentsAsync runs on every leader heartbeat (HealthCheckPollingTime, 10 seconds by default) for every registered store. It built the whole EventStoreUsage descriptor, which reads the highest event sequence and describes every projection, subscription, event type and tag type.

The issue says only usage.Subscriptions is used. That isn't quite right: the method also needs each database's server, database name and tenant ids, which drive the per-tenant agent fan-out. So this uses two narrower reads:

• Async shard names: IEventStore.RegisteredShardNames(), the store's in-memory registry, with no database work. It returns the same set the usage path produced: async projections plus every subscription, and subscriptions always report Async. A store that doesn't implement it falls back to the usage.
• Database topology: the new IEventStore.TryDescribeDatabasesAsync() from JasperFx 2.87.1 (IEventStore.TryDescribeDatabasesAsync: the database half of TryCreateUsage (wolverine#5002) jasperfx#1013). The tenant-to-database lookup, StartAllAsync and AllDaemonsAsync use it too.
• The registered-projection rebuild keeps the full usage, because it has to match Inline and Live projections as well.

Where the saving comes from

TryDescribeDatabasesAsync() is a default interface member that falls back to TryCreateUsage()?.Database. With the store versions Wolverine currently pins, this is correct but saves nothing yet. The saving is complete with Marten 9.50.1, Polecat 5.40.1 and Fisher 1.20.2, which override it to answer from their tenancy. Those releases are on their way, and an application can take them directly.

Tests

• Fake event stores in the agent tests now go through a shared StubUsage helper. NSubstitute intercepts default interface members too, so stubbing only TryCreateUsage would leave the new reads answering null and an empty list. This includes the test GH-5003 Resume continuous execution after a projection rebuild, even a failed one #5008 added.
• an_evaluation_builds_each_stores_usage_once now also asserts that an evaluation never calls TryCreateUsage.
• New: a_store_without_a_shard_registry_still_answers_its_shards_through_the_usage.

Verification

• dotnet build wolverine.slnx -c Release -f net9.0 against JasperFx 2.87.1: clean.
• CoreTests.Runtime.Agents (net9.0): 486 passed, 6 skipped, 0 failed.
• MartenTests.Distribution against local Postgres: 49 of 49 passed.

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