Support explicit single-tenant routing without synthetic TenantID

[fredcamaral]

Problem

lib-streaming currently rejects non-system events when EmitRequest.TenantID is empty. That is correct for SaaS multi-tenant deployments, but it leaves single-tenant/BYOC services with only bad choices:

• skip emitting business events when no tenant-manager tenant exists;
• put organizationId into TenantID, which is semantically wrong;
• invent a synthetic tenant UUID, which violates services that intentionally have no tenant identity in single-tenant mode.

In plugin-br-bank-transfer, we rejected the organizationId -> TenantID fallback because tenant identity and organization scope are distinct:

• tenantId comes from tenant-manager/JWT context and routes tenant-scoped infrastructure.
• organizationId is a business-scope identifier for CRM/Midaz/Fees calls.
• Overloading EmitRequest.TenantID with organization metadata contaminates CloudEvents/outbox routing metadata and can mislead downstream consumers.

The current safe plugin behavior is to skip emits when no real tenant is present. That preserves correctness but means single-tenant deployments lose streaming events.

Desired Capability

Add an explicit single-tenant routing mode that lets services emit non-system business events without providing a fake TenantID.

Possible API shapes:

1. Add a routing scope field:

type EmitRequest struct {
    DefinitionKey string
    TenantID      string
    RoutingScope  RoutingScope // e.g. MultiTenant, SingleTenant, System
    Subject       string
    EventID       string
    Payload       []byte
}

2. Add a dedicated single-tenant routing key/config at builder level:

streaming.WithSingleTenantRoutingKey("default")

3. Add an explicit method/helper:

emitter.EmitSingleTenant(ctx, EventRequest{...})

The important contract: the emitted event must not label this value as tenant identity (tenantId, ce-tenantid, outbox tenant_id) unless it is an actual tenant-manager tenant.

Acceptance Criteria

• Multi-tenant non-system events still fail fast when TenantID is missing.
• Single-tenant non-system events can be emitted through an explicit API/config path.
• CloudEvents/outbox metadata does not misrepresent organization IDs or synthetic IDs as tenant identity.
• Event manifests/documentation make the distinction between tenant identity and single-tenant routing metadata clear.
• Existing multi-tenant behavior remains backward compatible.

Downstream Context

This was found during Gate 8 review of plugin-br-bank-transfer. The branch temporarily considered using either organizationId or a fixed UUID as EmitRequest.TenantID; both were rejected because they violate the plugin tenant-isolation rules and produce misleading event metadata.

Until this is supported upstream, the plugin will skip emits when no real tenant exists rather than emit events with a false tenant identity.