PresenceMessage.memberKey not exposed (TP3h)

[paddybyers]

Spec point

• TP3h: `memberKey` is a "string function that combines the `connectionId` and `clientId` ensuring multiple connected clients with the same clientId are uniquely identifiable." It should be a method on `PresenceMessage`.

Observed behaviour

`memberKey` is not a method on `PresenceMessage`. It is computed internally as a lambda `(item) => item.clientId + ':' + item.connectionId` passed to `PresenceMap`, but not accessible to callers.

Failing test

Test	Expected	Actual
`TP3h - memberKey format`	`typeof msg.memberKey === 'string'`	`typeof msg.memberKey === 'undefined'`

Reproduction

```bash
RUN_DEVIATIONS=1 npx mocha --grep "TP3h" test/uts/rest/types/presence_message_types.test.ts
```

From PR #2191, branch `uts-rest`.

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