Implement Polecat primary-store database-per-tenant (GH-3445)

[jeremydmiller]

Closes #3445.

The gap

PolecatIntegration.MainDatabaseConnectionString was documented on the Polecat multi-tenancy page as the way to point Wolverine's envelope storage at a master database under database-per-tenant — but nothing read it. The primary IntegrateWithWolverine() path (WolverineOptionsPolecatExtensions.cs) unconditionally built a single non-tenanted SqlServerMessageStore from store.Options.ConnectionString, with no cardinality branch and no MultiTenantedMessageStore. Because it's a silent no-op rather than a compile error, a user following the docs shipped a system whose tenant messages went to the wrong store.

The PolecatMessageDatabaseSource / MultiTenantedMessageStore machinery already existed — but only on the ancillary path. Marten's primary path has always had the branch (WolverineOptionsMartenExtensions.cs:126-140).

The fix

Mirror the ancillary Polecat path and the Marten twin on the primary path:

• Dispatch on store.Options.Tenancy.Cardinality (not the tenancy type name — see the ancillary seam's marten#4864 note).
• For a multi-tenanted store, build a MultiTenantedMessageStore over a PolecatMessageDatabaseSource with a Role.Main master store for tenant-neutral state (nodes, assignments, dead letters), reading MainDatabaseConnectionString (falling back to the store's base connection string, throwing a clear error if neither is present).
• Single-tenant path is unchanged.

Tests

primary_store_database_per_tenant (SQL Server) asserts the primary store is now a MultiTenantedMessageStore and that durability agents exist for each tenant database. Verified red against origin/main, green with the fix.

Docs (items 2 & 3 from the issue)

• multi-tenancy.md: the sample called Marten's AddSingleTenantDatabase(conn, tenantId); corrected to Polecat's AddTenant(tenantId, conn) (reversed args).
• distribution.md: corrected the agent-URI example. Verified against the Polecat source: IEventStore.Identity => new(StoreName.ToLowerInvariant(), "SqlServer") and EventStoreIdentity(string Name, string Type), so UriFor emits {Type}/{Name} = sqlserver/main, not polecat/main.

Dependencies

Bumps Weasel 9.16.4 -> 9.17.0 and the Polecat floor to 5.1.0 (both latest on NuGet). Full wolverine.slnx builds clean in Release.

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