Django backend retains automatically opened client after send failure #38

Describe the bug

EmailBackend.send_messages closes a client it creates only when every batch succeeds. If send_batch raises, or a response contains a per-message failure, the backend retains the open client even when the caller did not explicitly open it. This affects both fail_silently settings.

To reproduce

With Django settings configured locally:

from unittest.mock import Mock, patch
from django.core.mail import EmailMessage
from postmark.django import EmailBackend

client = Mock()
client.outbound.send_batch.side_effect = RuntimeError("local failure")
with patch("postmark.django.backend.SyncServerClient", return_value=client):
    backend = EmailBackend(server_token="local-test", fail_silently=True)
    backend.send_messages([
        EmailMessage("example", "body", "sender@example.com", ["to@example.com"])
    ])
    assert client.close.called  # false
    assert backend.client is None  # false

Expected behavior

Close automatically created clients on failure as well as success. Preserve an explicitly opened/caller-owned connection for reuse. Keep exception and fail_silently behavior unchanged.

Environment

• postmark-python 0.4.0 / current main
• Python 3.12, Django 5.2, macOS

Additional context

The send loop and close call can be wrapped in try/finally inside the existing exception-handling boundary. I plan to cover transport errors and API response failures, both ownership modes and both fail_silently settings. Local mocked transport only; no actual emails sent. AI assistance: OpenAI Codex.