Fix MongoDB tenant-agnostic workflow definition persistence and lookup

[Copilot]

MongoDB multitenancy handling for workflow definitions was inconsistent: agnostic (TenantId == "*") definitions could be overwritten to null on write and excluded from tenant-scoped reads. This made CLR-defined workflows unreachable under concrete tenant contexts.

• Write-path tenancy behavior (MongoDbStore)

  • Preserve Tenant.AgnosticTenantId instead of restamping it.
  • Only apply ambient tenant stamping when TenantId is null.
  • Keep explicitly scoped tenant IDs unchanged.

• Read-path tenant filter (MongoDbStore)

  • Tenant-scoped queries now include agnostic documents in addition to tenant-matching ones.

• Regression coverage (Elsa.MongoDb.UnitTests)

  • Added targeted tests for:
    • preserving "*" on save,
    • stamping ambient tenant for unscoped entities,
    • including agnostic records in tenant-scoped filtering logic.

• Test assembly access

  • Added InternalsVisibleTo("Elsa.MongoDb.UnitTests") to expose internal filter logic for focused unit coverage.

internal static IQueryable<TDocument> ApplyTenantFilter(IQueryable<TDocument> queryable, string? tenantId) =>
    queryable.Where(x =>
        (x as Entity)!.TenantId == tenantId ||
        (x as Entity)!.TenantId == Tenant.AgnosticTenantId);

private static void ApplyTenantId(Entity tenantDocument, string? tenantId)
{
    if (tenantDocument.TenantId == Tenant.AgnosticTenantId) return;
    if (tenantDocument.TenantId != null) return;
    tenantDocument.TenantId = tenantId.EmptyToNull();
}

[arledesma]

If you merge this one then don't forget to close out #155

CoPilot also removed the reference to the source issue #158 from the description (it's available in history)