Fix SMTP-log integration fixtures and make EmailLogEntity.log optional

[ckenst]

The SMTP-log integration tests currently create messages using PostMessageRequest and immediately query their SMTP logs:

• GetMessageSmtpLogRequest.test.ts
• GetInboxMessageSmtpLogRequest.test.ts

Messages created through the HTTP API do not pass through SMTP. For these messages, the API returns a successful response with an empty string:
{ "log": "" }
Both tests expect result.log to be an array, so they fail:

Expected: true
Received: false`
expect(Array.isArray(result?.log)).toBe(true);

For a message genuinely delivered through SMTP, the API returns the expected array:

{
  "log": [
    {
      "log": "Connection from: 192.0.2.1",
      "time": "0",
      "event": "SOCKET_OPEN"
    },
    {
      "time": "69",
      "event": "TLS_ACTIVE"
    },
    {
      "time": "174",
      "event": "SOCKET_CLOSED"
    }
  ]
}

This also shows that individual SMTP-log entries do not always include the inner log property. The current SDK model incorrectly declares it as required:

export class EmailLogEntity {
    log: string;
    time: string;
    event: string;
}

Expected behavior

The integration tests should query a known message that was delivered through SMTP, and the SDK type should accurately represent entries where the textual log value is absent.