[jeremydmiller]

Fixes #5297

Adds DbContextDocumentStoreDiagnostics<T> (registered next to each DbContextUsageSource, subject efcore://{Context}), so a tool can browse a DbContext's entities through IDocumentStoreDiagnostics.

• Reads only. Set<T>().AsNoTracking() + ApplyCriteriaTo; translation failures become DocumentCriteriaNotSupportedException. Ordered by primary key when no ordering is given. Load by single-column key; keyless/composite-key types are listed and pageable, load returns null, IdEquals is refused.
• Scalar properties only, by property name; no navigations, no Include. Owned and shared-type entities aren't listed.
• Wolverine's own entities: WolverineMappedEntities.IsWolverineEntity(...) identifies IncomingMessage/OutgoingMessage. DocumentTypeRef has no flag for it, so this is the helper.
• Tenancy: multi-tenanted contexts are built through IDbContextBuilder<T> (conjoined pins the tenant so the query filter scopes it; database-per-tenant picks the tenant database). AllTenants is refused on those (honouring it on conjoined means IgnoreQueryFilters, which would bypass other filters too); AllTenants + TenantId throws ArgumentException.
• Refused: OrderByMetadata, correlation/causation/last-modified filters (no such columns).
• Explain: returns the SQL EF Core generates for the page (ToQueryString). A provider-native plan was not done: ToQueryString leaves parameters unbound, so it can't be fed to EXPLAIN as is. Left as a follow-up.
• Query-filter soft deletes are the filter's business; IncludeSoftDeleted is not interpreted.

Tests: efcore_document_store_diagnostics (SQL Server, 12) and conjoined_document_store_diagnostics (Postgres conjoined, 5) pass locally.

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