Expand channel and retry edge coverage

[mic1on]

Summary

• add Email edge tests for missing/invalid ports, implicit STARTTLS, disabled STARTTLS, async missing recipients, and SMTP close fallback
• add provider response validation tests for non-dict payloads, dict error details, and fallback payload details
• add Publisher retry and async failure aggregation edge tests
• align async Email missing-recipient log output with the documented to_emails field

Closes #48

Verification

• make lint
• make test (116 passed)
• make coverage (97% total)
• uv build
• uv pip check