Function createCompositeBackend

createCompositeBackend(
resolveEnabled: () => readonly EnabledBackend[],
getDefault: () => string | undefined,
): MessagingBackend
Build one MessagingBackend that spans every enabled provider.

Conversation and message ids leaving the composite are namespaced as "<providerId>:<innerId>"; ids arriving from callers are split on the first : to find the provider. Pagination cursors for conversations() are namespaced the same way and are only produced when exactly one provider is involved; with several providers the merged page carries no cursor, because no single provider can resume it.

close() is a no-op: the registry that enabled the backends owns their lifetime and closes them through MessagingRegistry.close(). Unsubscribing a composite subscription skips providers that were disabled in the meantime, since disabling already closed their backend.
Parameters
- resolveEnabled: () => readonly EnabledBackend[]
  Returns the currently enabled providers, in order.
- getDefault: () => string | undefined
  Returns the id of the provider used for sends that name none.
Returns MessagingBackend
A backend suitable for createDmService.
Example
```
const dm = createCompositeBackend(
  () => [{ id: 'private', backend: privateBackend }],
  () => 'private',
);
const page = await dm.conversations({ limit: 20 });
page.conversations[0]?.id; // 'private:direct:ab,cd'
```